In 5 years, Dad will be three times as old as his daughter Jill will be then. If the sum of their present ages is 50, how old are they now?

If x + 5 is Jill's age five years from now, which of the following equations could be used to solve the problem?

a. 3x + 5 = 55 - x

b. 3x + 15 = 55 - x

c. 3x + 15 = 45 - x ... ?

The answer is b. 3x + 15 = 55 - x

x - Jill's present age

y - dad's present age

In 5 years, Dad will be three times as old as his daughter Jill will be then:

y + 5 = 3 (x + 5)

The sum of their present ages is 50:

x + y = 50

We have the system of two equations now:

y + 5 = 3 (x + 5)

x + y = 50

Let's rearrange the second equation:

If: x + y = 50

Then: y = 50 - x

Now, substitute y from the second equation (y = 50 - x) into the first one:

y + 5 = 3 (x + 5)

50 - x + 5 = 3 (x + 5)

50 + 5 - x = 3*x + 3*5

55 - x = 3x + 15

Rearrange it a bit:

3x + 15 = 55 - x

Therefore, the correct choice is b.