Question 397646
Let b and m be Bob's and Mary's ages. Then,


b-6 = 4(m-6) --> b-6 = 4m - 24


b+4 = 2(m+4) --> b+4 = 2m + 8


This is essentially a system of equations that can be solved for b and m. Solving using any method (I used substitution), we get m = 11, b = 26, so Mary is 11 and Bob is 26.