Adding and subtracting seem easy, but care must be taken that the units involved are the same. If they are not the same, they must be converted. This becomes the basic notion needed in the next activity concerning adding and subtracting fractions.

Using Number Properties

Problem: “Add $4.34 and 108 cents in two ways.”
(Answer: Convert the cents into dollars and cents so $4.34 + $1.08 = $5.42. Convert dollars and cents into cents, so 434 cents + 108 cents = 542 cents.)

Problem: “Find 109 cm – 80 mm.”
(Answer: 1 090 mm – 80 mm = 1 010 mm or 109 cm – 8 cm = 101 cm.)

Examples: Work these problems out in two different ways: 100 mm + 1.5 m
5.6 kg – 700 g
2 min 30 sec + 40 sec
2 hrs 30 min + 50 min

Example: $US1 = $NZ2. Fiona has $US10 and $NZ30 in her purse. How much money does she have altogether in either currency?

Understanding Number Properties:

Make up an addition or subtraction problem with a mixture of millilitres and litres and solve the problem in two ways.