Average of total books sold by stores B and C together is how much more than total unsold books of store A.

Average of total books sold by stores B and C

$= frac{1}{2}left( {50 imes frac{{40}}{{100}} imes 100 + 45 imes frac{{30}}{{100}} imes 100} ight)$

= 1675

Unsold books of store A = $3500 imes frac{{80}}{{100}} = 2800$

Required difference = 2800 – 1675 = 1125