Question 246125
Let x = Mary's age
Let y = Joan's age
Let x + y  = 15

Solve for y:

y = 15 - x



Translate the words into algebra:

Twice Joan's age = 2y or 2(15-x)
3 times the sum of 5 and Mary's age = 3(5 + x)

Set these two expressions equal to each other:

2(15 - x) = 3(5 + x)

Solve:

30 - 2x = 15 + 3x

15 = 5x

x = 3 

15 - x = y = 12


Mary's age is 3 
Joan's age is 12

This solution also satisfies teh condition that Mary be younger than Joan