Need working

Each pen cost 90¢ less than each notebook.
So each notebook cost 90¢ more than each pen.

If we let cost of 1 pen = 1 unit,
then cost of 1 notebook = 1 unit + 90¢

cost of 5 note books = 5 units + 5 × 90¢
= 5 units + 450¢
= 5 units + $4.50


Since he spent twice as much on 5 note books as on 7 pens,


cost of 5 notebooks = 2 × cost of 7 pens
cost of 5 notebooks = cost of 14 pens
cost of 5 notebooks = 14 units


So 5 units + $4.50 = 14 units

$4.50 = 9 units

1 unit = $4.50 ÷ 9 = $0.50


So, cost of 1 pen = $0.50
Cost of 1 notebook = $0.50 + $0.90 = $1.40