In mathematics, 'is' would be the equal sign. And 'of' would be the multiplcation sign. The missing number would be any variable.
56 is 28% of a number would be translated algebraically to:

Solving for x, we get:

Divide 28 on both sides

Multiply 100 on both sides
56 is 28% of 200
Hi!
In my opinion, the easiest way to solve a problem like this is to find the greatest common factor (GCF) of the numerator and denominator and then divide both numbers by the GCF.
So first we need to find the factors of 45 and 72 and find the factor that has the most value that the two numbers both have.
45: 1, 3, 5, 9, 15, and 45
72: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72
It looks like the GCF is 9. Now we have to divide 45 and 72 by 9.
45 ÷ 9 = 5
72 ÷ 9 = 8
So the correct answer should be:

Hope this helps :)