In order to calculate answers to questions involving time, you must know which units are being used. In questions referring to a week, please note that it is a school week of 5 days.
When working out children's reading ages it is necessary to consider time given in years and months. A child may have an actual age of 8 years and 4 months, but a reading age of 8 years and 10 months. Another child of the same age might have a reading age of 7 years and 9 months. Reading ages are presented in several forms: 8 years 6 months, 8.6 or 8-6 are all common. If the form 8.6 is used, it has to be remembered that here the point is not a decimal point, just a means of separating months and years.
Example one
A junior school is putting on a school entertainment. If each of the four classes is allowed 25 minutes and the entertainment starts at 2.15pm, when is it expected to finish?
If each class were allowed half-an-hour, the entertainment would finish 2 hours after the start, ie 4:15pm.
Each class has 25 minutes, which is 5 minutes less than half-an-hour.
So four classes would take 4 x 5 minutes less – ie 20 minutes less.
The entertainment should therefore finish 20 minutes before 4.15pm. 15 minutes before 4.15pm is 4.00pm, 20 minutes before 4:15pm is 3:55pm.
An alternative method would be to calculate:
25 minutes x 4 = 100 minutes = 1 hour 40 minutes
So the four classes take 1 hour 40 minutes.
1 hour 40 minutes after 2.15pm is 3.55pm.
Example two
A parent comes in to a school for one-and-a-half hours to hear a group of children reading. If there are 10 children in the class, can she give them all 10 minutes each?
One-and-a-half hours is 60 + 30 = 90 minutes.
To give 10 children 10 minutes each would take 100 minutes. So she cannot give each of them 10 minutes.
Avoiding common errors
Most common errors can be avoided by:
- checking the units used in the question
- not using units of time as if they were decimals - for example, when referring to reading ages 7.5 does not mean 7 years (7 years 6 months), but 7 years 5 months. To avoid confusion, the notation 7-5 is often used
- being clear about am and pm on the 12-hour clock, and
- taking care with times between 12:00 and 24:00 on the 24-hour clock.