Let envelopes be x and postcards be y.
Given that: X = 3.5 each and y = 1.25 each and she spent an amount of 20 - 2.25 (change she got back) = 17.75
So what we now have is: 3.5x + 1.25y = 17.75
So y (no of postcards) = (17.75 - 3.5x) / 1.25
Now plug in the answer choices and you should have your answer.
Lets see if x = 1, what is y? as per:
y = (17.75 - (3.51)) / 1.25 = 11.4 (which cannot be correct as it is not an integer)
Now let’s try x = 3; y = (17.75 - (33.5)) / 1.25 = 5.8 (again not correct as it is not an integer)
What if x = 4? y = (17.75 - (4*3.5)) / 1.25 = 3! (an integer!)
Hence no of postcards (y) is 3.
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