Hey there!
Tom's age to Mary's is 3:4; an equivalent ratio is 15:x (x symbolizes Mary's age).
To solve for this, we need to divide our proportions. By that, I mean divide Tom's older age by his younger age. If we do so, we should get a scale factor that we can multiply Mary's younger age by.
15 / 3 = 5
This means that the scale factor of the ages is 5.
x / 4 = 5 (Solving for Mary's unknown age.)
To solve for x, multiply both sides by 4.
x/4 * 4 = x
5 * 4 = 20
x = 20
Mary's age is 20.
I hope this helps!
