1/b* a = a/b

When a fraction is multiplied by a number, it's the same thing as multiplying the numerator by that number. If the numerator is 1 it's multiplied by 1, and thus, doesn't change its value. 1/3* 2 = 1* 2/3=2/3 This can be seen as the factor replacing the numerator if the numerator is 1.