The canvases cost $40.00 and she spent half that amount on paint so she spent 40/2 = $<<40/2=20.00>>20.00 on paint
She spent $40.00 on canvases, $20.00 on paint and $15.00 on an easel so she spent 40+20+15 = $<<40+20+15=75.00>>75.00
She spent $90.00 total and her other purchases totaled $75.00 so she spent 90-75 = $<<90-75=15.00>>15.00 on paint brushes
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