Question 1059463
Let Mark's older son be m, and his younger son be n. Then
m+n=51
m=51-n
and
m-21=2(n-21), So:
51-n-21=2n-42
3n=72
n=24
Let p be the number of years ago that m and n together made half his age. Then
51-p=2(24-p+27-p)
51-p=48+54-4p
3p=51
p=17
17 years ago, Mark was twice as old as his sons combined. ☺☺☺☺