Question P6 Constant difference
Today’s problem — free all day, every step. Every day has one.
The ratio of Wei's age to his father's age is 1 : 4. The difference between their ages is 30 years. In how many years will the ratio of Wei's age to his father's age be 2 : 5?
Answer
10 years.
The gap between the two amounts never changes.
In this one. An age gap is fixed for life, so the difference is the quantity to measure both ratios against — 3 units then, 3 parts now, the same 30 years.
Both amounts go up or down by the same amount. Ages are the common case, because the same number of years passes for both people.
Children treat the gap as fixed when only one person changed, forgetting that adding to one person alone moves the gap by exactly that amount.
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