Minutes and hours in rate questions: the 2.3 hours mistake
On the script you see 2 h 30 min written as 2.3 h, and the rate that follows is wrong. Your child knows a rate is an amount per hour; the slip is treating an hour as if it had 100 minutes. One wrong time spoils every later step that uses it.
What it looks like
A tap fills a tank with 115 litres of water in 2 h 30 min. At the same rate, how many litres of water does it fill in 1 hour?
- 30 min = 30/60 h = 0.5 h
- 2 h 30 min = 2.5 h
- Water in 1 hour = 115 ÷ 2.5 = 46 litres
Why 50 litres is wrong: 30 min was written as 0.3 h, so the time became 2.3 h and 115 ÷ 2.3 = 50.
Why children do it
- Money and length use 100 (100 cents, 100 cm), so dividing by 100 feels natural.
- The written form "2 h 30 min" looks like a decimal, so children copy it straight across as 2.3.
- Calculators accept 2.3 without complaint, so nothing looks wrong on the page.
How to fix it at home
- Make "minutes over 60" a rule: write 30 min as 30/60 h before turning it into a decimal.
- Learn the common ones by heart: 15 min = 0.25 h, 30 min = 0.5 h, 45 min = 0.75 h, 20 min = 1/3 h.
- Ask your child to check the answer by multiplying back: 46 × 2.5 should give the 115 litres.
- Practise a few rate questions where the time is given in hours and minutes, not whole hours.
Questions parents ask
Why does my child get rate questions right sometimes and wrong at other times?
Often the right ones use whole hours. The mistake only shows when the time has minutes in it, such as a tap, a machine or a worker over 1 h 45 min.
Should my child use fractions or decimals for time?
Either works. Fractions such as 1/3 h avoid rounding when the minutes do not divide neatly, so 20 min and 40 min are easier as fractions.
Where it shows up
Other careless mistakes
Stop losing marks to the same mistake.
Three PSLE questions, free, no sign-up. You’ll see exactly where the marks went. Then 7 days of everything for S$4.99.