Mark is three times as old as his sister: M = 3S
Two years ago, he was seven times as old as his sister: M - 2 = 7(S2)
Substitute the value of M from the first equation into the second equation:
3S - 2 = 7(S - 2)
Now distribute the 7 on the right side:
3S - 2 = 7S - 14
Subtract 3S from both sides:
-2 = 4S - 14
Add 14 to both sides:
12 = 4S
Divide by 4:
S = 3
Now that we have the sister's age, we can find Mark's age using the first equation:
M = 3S
M = 3 * 3
M = 9
Answer: Mark 9 years old; sister 3 years ols