Capacity of G + M = 720 / 20 = 36

M + H = 720 / 24 = 30

H + G = 720 / 15 = 48

Combined capacity = 2 (G + H + M) = 114

G + H + M = 114 / 2 = 57

Now capacity of G = (G+H+M) - (H + M) = 57 - 30 = 27

M = (G+H+M) - (H + G) = 57 - 48 = 9

H = (G+H+M) - (G + M) = 57 - 36 = 21

Given that G worked for 4 days, and mark for 8 and harry for 8 days

So total work by them = 4 x 27 + 8 x 9 + 8 x 21 = 348