Answer:
Andrew's age is 8
Mary's age is 4
Step-by-step explanation:
Let Andrew's age be represented by x
Since he is 4 years older than his sister, his sister's age is (x - 4)
The sum of their ages is 20, this means
x + (x - 4) = 20
Solving for x from the above equation, we can obtain their ages
x + x - 4 = 20
2x = 20 - 4
2x = 16
x = 16/2 = 8
Andrew's age is 8
Mary's age is 8 - 4 = 4