Theory

a* 1/b = a/b

When a number is multiplied by a fraction, 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. 2* 1/3 = 2* 1/3=2/3 This can be seen as the factor replacing the numerator if the numerator is 1.
Exercises