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Wei Yang's money box — 20c and 50c coins
Wei Yang started a savings plan by putting 2 coins in a money box every day. Each coin was either a 20c or 50c coin. His mother also put in a $1 coin in the box every 7 days. The total value of the coins after 182 days was $133.90.
- (a) How many coins were there altogether?
- (b) How many of the coins were 50c coins?
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The pattern behind it: Suppose and swap
Suppose everything is one kind, then swap one at a time until the total is right.
In this one. When a total mixes coins you can count exactly with coins you cannot, take the countable ones out first — then the rest is a plain "suppose they are all the cheap one" problem, and each swap to the dearer coin buys back a fixed difference.
How to spot it
Two kinds of thing are mixed together with a known total count and a known total of something else, such as legs or sweets, and neither kind's count is given on its own.
Common mistake
Children lose track of whether a swap adds to the total or takes away from it. Compare the starting total with the target first, then swap in the direction that closes the gap.
Try one like it
Standard Bicycles and tricycles — 58 wheels A cycling club has 25 vehicles parked outside, a mix of bicycles and tricycles. Bicycles have 2 wheels and tricycles have 3 wheels…The 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.
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