Answer:
21 to 28
Step-by-step 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 = (53) : (54) = 3 : 4
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21 : 28 = (73) : (74) = 3 : 4 . . . . same as 15 : 20
5 : 10 = (51) : (52) = 1 : 2 . . . . . ≠ 3:4
18 : 25 . . . has no common factors . . . ≠ 3:4
