Answer:
21 to 28
Explanation:
Reduce each ratio to lowest terms by dividing by any common factors. (It helps to know your multiplication tables, or a quick way to find the greatest common divisor.)
15 : 20 = (5*3) : (5*4) = 3 : 4
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21 : 28 = (7*3) : (7*4) = 3 : 4 . . . . same as 15 : 20
5 : 10 = (5*1) : (5*2) = 1 : 2 . . . . . ≠ 3:4
18 : 25 . . . has no common factors . . . ≠ 3:4