determine this equivalents fractions to?

Question

I need someone to explain me how to do this 

2/3 y 3/5 con denominator 15

1/2, 3/4 y 5/6 con denominator 12

1/4, 2/3 y 7/8 con denominator 24

Answer 1

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10/15 y 9/15.

6/12, 9/12, y 10/12.

6/24, 16/24, y 21/24.

Answer 2

3 * ? = 36.  --> ? = 12.

3*12 / (5 * 12) = 36 / 60.

.U can now do the others...