Answer:
15
Step-by-step explanation:
So if we look at 7 and 21, they are both in the same position in the ratio. So first we look for the relationship. 7 goes into 21 three times, so that's our basis for the equivalent. If we start at 5, we would have to find what 5 goes into three times, being 15.
So, 7:5 is equivalent to 21:15
