First Column:(Left handed)
BOys + girls = 29
17 + x = 29
x = 29 -17
x= 12
girls = 12
3rd column ( Total )
Boys + girls = 70
35 + x = 70
x = 70 - 35
x= 35
Total girls 35
3rd Row :(Total )
Left handed + Right handed = 70
29 + x = 70
x = 70-29
x= 41
Total right handed : 41
2nd column (Right handed)
Boys + girls = 41
x + 23 = 41
x= 41-23
x= 18
Right handed boys = 18