Today’s problem — free all day, every step. Every day has one.
Stamp collectors — Nadia and Farid
Nadia and Farid are stamp collectors. Each of them has 150 stamps in his or her album, made up only of 20-cent stamps and 60-cent stamps. Nadia has 18 more 60-cent stamps than Farid. Farid has 40 sixty-cent stamps in his album. Each 60-cent stamp weighs 3 g and each 20-cent stamp weighs 2 g.
- (a) Whose stamps are worth more in total value, and by how much?
- (b) What is the total weight of Farid's stamps, in kilograms?
Show the answerHide the answer
See all 11 steps
MathBrush is being tested with a few parent groups, and every solution on the site is open to them — one household, all children. If you were given a code, it goes in once and this device remembers it.
I have a code → See the free ones
65 worked examples are free without a code, two from every pattern. No account, either way.
The pattern behind it: Repeated identity
One length or count, described two ways in the same question.
In this one. Because both albums contain the same number of stamps, cancel the matching stamps: only the 18 swaps from a 20-cent stamp to a 60-cent stamp can change the value.
How to spot it
A shape or count is described from two viewpoints in the same question, such as one edge measured as a number of long pieces and again as a number of short pieces.
Common mistake
Children treat the two descriptions as separate quantities and add them. Both describe the same quantity, so the two must be equal.
Try one like it
Challenging Coins — Helen and Ivan Helen and Ivan have the same total number of coinsThe 33 problem patterns
All 33 patterns, one to a page — the 19 behind Primary 5 and 6 word problems, and the skills the younger years are built on. The bar model, how to spot it in the question, and the mistake children make. A booklet you can print. Free.
A PDF, and a note when new solutions go up.