Answer:
10 days
Step-by-step explanation:
The problem seems to assume that Manuel will continue to buy postcards at exactly the rate of 2 per day during his whole vacation. 2×10 = 20, so it will take 10 days to accumulate 20 postcards.
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If you like proportions, you could write one:
days/postcards = 3/6 = ??/20
Multiplying by 20 gives ...
20(3/6) = ?? = 10 . . . . . 10 days on vacation will result in 20 postcards
