Question 1005759
Call the numbers x and y.  Then we have
x - y = 15  and  
(1/3)(x + y) = (1/4)x
Let's clear the fractions on the second one, by multiplying by twelve...
4(x + y) = 3x
4x + 4y = 3x
x + 4y = 0
If we subtract the first equation from this we get
x + 4y = 0
-(x - y = 15)
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5y = -15
y = -3
x is then 12.