Measurement stories mix everything you have learned: converting units, whole-number operations, and fractions or decimals. The key is to work in one unit.
Key idea
First put every amount in the same unit (often the smaller one). Then decide on the operation and solve. For time, count up in friendly jumps to the next hour. For money, it can help to think in cents.
Why it works
You can only add or subtract amounts that are measured the same way, just as you can only add fractions with like pieces. 2 kilograms minus 650 grams makes sense once both are grams: $2{,}000 - 650 = 1{,}350$ grams.
Worked example: Elapsed time
Practice starts at 3:45 p.m. and ends at 5:10 p.m. How long is practice?
- From 3:45 to 4:00 is 15 minutes.
- From 4:00 to 5:00 is 60 minutes.
- From 5:00 to 5:10 is 10 minutes.
- Add: $15 + 60 + 10 = 85$.
Answer: 85 minutes
Worked example: Money
Ava pays for a \$4.35 sandwich and a \$1.50 juice with a \$10 bill. How much change does she get?
- Think in cents: \$4.35 is 435 cents and \$1.50 is 150 cents. Total: $435 + 150 = 585$ cents.
- \$10 is 1,000 cents. Change: $1{,}000 - 585 = 415$ cents, which is \$4.15.
Answer: \$4.15
Worked example: Mass in two units
A bag holds 2 kg of flour. A baker uses 650 g. How much is left?
- Change to grams: 2 kg = 2,000 g.
- $2{,}000 - 650 = 1{,}350$.
Answer: 1,350 grams