USE multiplying by 1 to find and expression equivalent to 15

USE multiplying by 1 to find and expression
equivalent to 15/17 with a denominator of 136a.
If I multiply by 1 would it still not be 15/17??? So I am confused???

Solution

(15/17)= ?/136a
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(15/17)(136a/136a) = (15*136a/17*136a) = (15*8a/136a) = 120a/136a