Question P6 Constant part
Today’s problem — free all day, every step. Every day has one.
A stationery shop had 200 more pencils than pens. After 100 pens were sold, the ratio of the number of pens to the number of pencils was 2 : 5. How many pencils were there?
Answer
There were 500 pencils.
The ratio changes, but one person's amount stays the same.
In this one. The pencils were never sold, so the gap between the two can only grow — it grows by exactly the 100 pens that left, which is why 200 + 100 = 300 is the value of the ratio's unit-gap.
A ratio changes and the question tells you only one person gained or lost an amount, while the other kept what they had.
Children keep the same number of shares for the person whose amount did not change. The amount stays the same, but the size of a share can differ between the two ratios.
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