Answer:
Mary is 34 years old, and Dad is 34 + 31, or 65, years old.
Explanation:
Represent the ages of Dad and Mary by d and m respectively.
Write and solve a system of linear equations as follows:
d = 2m - 3
d - m = 31
Substituting 2m - 3 for d in the second equation, we get:
2m - 3 - m = 31
Combining like terms results in
m = 34. Mary is 34 years old, and Dad is 34 + 31, or 65, years old.