3/4 = 21/28 after multiplying top and bottom of 3/4 by the value 7
3*7 = 21 and 4*7 = 28
The goal is to get 3/4 into an equivalent fraction that has 28 as the denominator.
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So 3/4 - 1/28 is the same as 21/28 - 1/28
The denominators are the same. We subtract the numerators to get 21-1 = 20. The denominators stay the same at 28
We have
21/28 - 1/28 = (21-1)/28 = 20/28
Now reduce 20/28 as much as possible. Divide both parts by the GCF 4
20/4 = 5
28/4 = 7
The final answer is 5/7