The first way is to cross multiply
4/5 = 28/x
4x = 5*28
4x = 140
4x/4 = 140/4
x = 35 which is the answer
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The second method is to note the denominators 4 and 28, and to ask yourself "what times 4 gets me 28?". The answer being 7, so 7*4 = 28. If you multiply top and bottom of the fraction 4/5 by 7, then you end up with 7*4 = 28 up top and 7*5 = 35 down below. This is another way of seeing how 4/5 = 28/35
Either way, the answer is x = 35
