Joanna bought only $0.15 stamps and $0.29 stamps. How many $0.15 stamps did she buy?

1) She bought $4.40 worth of stamps.

I rephrased this question to

15x + 29y = 440.

I think its just trial and error and applying all your knowledge of math...

for eg., we know that she bought .15 and .29 stamps and no other denominations.

so is there a combination that gives 4.40 from .15 and .29 , yes 10 and 10; we have to make sure that there is no other solution to get to 4.40 using .15 and .29

if we try 11 for 0.15 then it is 1.65 and the rest 2.75 is not a multiple of 0.29 , so must not be 11 0.15 stamps.

15x +29y=440, we know x and y are atleast 1

lets try

x=1 then y must be (440-15)/29 and the result is not an integer so x must not be 1

trying in this we we only find x=10 and y=10 to be a solution and other combinations dont work.... hence it has to be x=y=10

I am sorry, I too dont have a better way to answer this..Well I just tried...

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Thanks, Sri

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