Find difference between average number of books sold by store R & S on 15 April, 2015 and average number of books sold by store Q & U on 15 April, 2015?

Average number of books sold by store R & S on 15 April, 2015

$= frac{{120 + 90}}{2}$ = 105

Average number of books sold by store Q & U on 15 April, 2015

$= frac{{60 + 100}}{2} = 80$

Required difference = 105−80=25