Mary paid $42.50 for 3 diaries and 5 pens yesterday. Today she paid $55 for 4 similar diaries and 6 pens. How much does each item cost separately?

let the cost of pens be x

let
the cost of diaries be y

then on the first
day,3y+5x=$42.50      (equation 1)

on second
day,4y+6x=$55                     (equation 2)

on solving 1
and 2 ,we get :y=10;x=2.50

thus,the cost of pens
=$2.50

the cost of diaries =$10