شرح حسابات الزمن

Worked example

1. 1

   $08:45\to 09:00=15\text{ min}$ — Count up to the next whole hour first. Whole hours are easy to count from.

2. 2

   $09:00\to 14:00=5\text{ hours}$ — Now jump in whole hours: from 9 to 14 is 14 − 9 = 5 hours.

3. 3

   $14:00\to 14:15=15\text{ min}$ — The end time is 14:15, so 15 minutes are left over.

4. 4

   $15+15=30\text{ min}$ — Add the two minute jumps. 30 is less than 60, so it stays as minutes.

5. 5

   $5\text{ hours }30\text{ minutes}$ — Put the hours and the minutes together. That is how long it took.

Worked example

1. 1

   $21:50\to 22:00=10\text{ min}$ — Count up to the next whole hour, as before.

2. 2

   $22:00\to 24:00=2\text{ hours}$ — Count on to midnight. On the 24-hour clock midnight is 24:00.

3. 3

   $00:00\to 06:00=6\text{ hours}$ — Midnight is also 00:00, the start of the new day. Count on to 06:00.

4. 4

   $2+6=8\text{ hours}$ — Add the two hour jumps.

5. 5

   $06:00\to 06:15=15\text{ min}$ — The minutes left over at the end.

6. 6

   $10+15=25\text{ min}$ — Add the minute jumps. 25 is less than 60, so it stays as minutes.

7. 7

   $8\text{ hours }25\text{ minutes}$ — Put the hours and minutes together.

Worked example

1. 1

   $4:35\text{ pm}\to 4+12=16:35$ — pm means after midday, and midday is 12:00. So add 12 to the hours. The minutes do not change.

2. 2

   $19:50\to 19-12=7:50\text{ pm}$ — The hour is more than 12, so it is after midday. Take 12 away from the hours and write pm.

3. 3

   $09:10\to 9:10\text{ am}$ — The hour is less than 12, so it is morning. Keep the numbers, drop the 0 at the front and write am.

4. 4

   $00:20\to 12:20\text{ am}$ — 00 is the first hour after midnight. On the 12-hour clock that hour is called 12, so write 12:20 am.

Worked example

1. 1

   $35+50=85\text{ min}$ — Add the minutes-only parts first: 35 and 50.

2. 2

   $85\text{ min}=1\text{ h }25\text{ min}$ — 85 is more than 60, so take 60 minutes out as 1 hour. 85 − 60 = 25 minutes are left.

3. 3

   $1\text{ h }25+1\text{ h }20=2\text{ h }45$ — Add the part with hours: 1 + 1 = 2 hours, and 25 + 20 = 45 minutes.

4. 4

   $10:40+2\text{ h}=12:40$ — Count on from the start time. Do the 2 whole hours first.

5. 5

   $12:40+45\text{ min}=13:25$ — Then the 45 minutes: 20 minutes takes you to 13:00, and 25 more makes 13:25.

Worked example

1. 1

   $48+25=73\text{ s}$ — Add the seconds first, just like the minutes in the last example.

2. 2

   $73\text{ s}=1\text{ min }13\text{ s}$ — 73 is more than 60, so take 60 seconds out as 1 minute.

3. 3

   $3+5+1=9\text{ min }13\text{ s}$ — Now add the minutes, plus the 1 minute carried from the seconds.

4. 4

   $15\text{ min}-9\text{ min }13\text{ s}$ — The songs must fill 15 minutes, so take away the time already used.

5. 5

   $9\text{ min }13\text{ s}\to 10\text{ min}=47\text{ s}$ — Count on to the next whole minute: 60 − 13 = 47 seconds.

6. 6

   $10\text{ min}\to 15\text{ min}=5\text{ min}$ — Then count on in whole minutes up to 15.

7. 7

   $=5\text{ min }47\text{ s}$ — Put the two jumps together. That is the time left for the last song.

Worked example

1. 1

   $08:15\to 11:15=3\text{ h}$ — Count on in whole hours first: 11 − 8 = 3 hours.

2. 2

   $11:15\to 11:45=30\text{ min}$ — Then the minutes left: 45 − 15 = 30 minutes.

3. 3

   $3\text{ h}=3\times 60=180\text{ min}$ — 1 hour is 60 minutes, so multiply the hours by 60.

4. 4

   $180+30=210\text{ min}$ — Add the 30 extra minutes. The whole time is now in one unit.

5. 5

   $210\times 60=12600\text{ s}$ — The rate is in seconds, so change the time to seconds too: 1 minute is 60 seconds.

6. 6

   $12600÷6=2100\text{ bottles}$ — One bottle every 6 seconds, so find how many 6s fit into 12 600: divide.

Worked example

1. 1

   $2.25\text{ hours}=2\text{ h}+0.25\text{ h}$ — Split off the whole hours. Only the decimal part needs changing.

2. 2

   $0.25\times 60=15\text{ min}$ — The decimal part is a part of one hour. An hour is 60 minutes, so multiply it by 60.

3. 3

   $2\text{ hours }15\text{ minutes}$ — Put the whole hours back with the minutes.

Worked example

1. 1

   $15+25+35=75\text{ min}$ — Add all the parts first, so you only count back once.

2. 2

   $75\text{ min}=1\text{ h }15\text{ min}$ — 75 is more than 60, so take 60 minutes out as 1 hour.

3. 3

   $08:20-1\text{ h}=07:20$ — Count back the whole hour first.

4. 4

   $07:20-15\text{ min}=07:05$ — Then count back the 15 minutes: 20 − 15 = 5.

5. 5

   $07:05+1\text{ h }15\text{ min}=08:20$ — Check by counting forward. It lands on 08:20, so 07:05 is right.

Worked example

1. 1

   $22:15-2\text{ h}=20:15$ — Count back the whole hours first.

2. 2

   $20:15-15\text{ min}=20:00$ — Count back 15 minutes to reach the whole hour.

3. 3

   $20:00-25\text{ min}=19:35$ — The rest of the 40 minutes is 40 − 15 = 25. Before 20:00 means 19-something, and 60 − 25 = 35.

4. 4

   $19:35+2\text{ h }40\text{ min}=22:15$ — Check by counting forward: it ends at 22:15, as the question says.

Worked example

1. 1

   $09:25\to 09:48=23\text{ min}$ — Use Bus 1, the full column. Northgate to Market Square takes 48 − 25 = 23 minutes.

2. 2

   $09:48\to 10:12=24\text{ min}$ — Market Square to Riverside: 12 minutes up to 10:00, then 12 more, so 24 minutes.

3. 3

   $11:03+24\text{ min}=11:27$ — Bus 2 needs its Riverside time. Every bus takes the same time, so add 24 minutes: 3 + 24 = 27.

4. 4

   $13:55+23\text{ min}=14:18$ — Bus 3 needs its Market Square time. 5 minutes takes you to 14:00, then 18 more.

5. 5

   $14:18+24\text{ min}=14:42$ — Check against the time already printed. 14:18 plus 24 minutes is 14:42, which matches.

6. 6

   $10:35\to 11:03=28\text{ min}$ — You reach Market Square at 10:35, after Bus 1 has gone. The next bus there is at 11:03: 25 minutes to 11:00, then 3 more.

Worked example

1. 1

   $22:40+7\text{ h}=05:40$ — Add the flight's whole hours first. 22 + 7 = 29, and 29 − 24 = 5, so it has passed midnight: 05:40 on Tuesday.

2. 2

   $05:40+25\text{ min}=06:05$ — Then the minutes: 20 minutes to 06:00, then 5 more.

3. 3

   $06:05\text{ Dubai time}$ — This is still Dubai time, because we started from the Dubai clock.

4. 4

   $06:05+4\text{ h}=10:05$ — Singapore is 4 hours ahead, so its clocks show a later time. Add 4 hours.

5. 5

   $10:05\text{ on Tuesday}$ — The landing time in Singapore. Give the day too, because the question asks for it.