Drop Down box linked to a total

My goal is to create dropdown boxes that add and multiply together to make a total. The idea is to have workout exercises each cost a specific amount (example: 1 exercise cost 3 dollars). The client can choose how many exercises they would like in a day (example: 3 exercises in 1 day = 9 dollars). Next dropdown box will allow the client to choose how many days per week they would like to workout (example: 2 days per week, 9 x 2 = 18 dollars). Then choosing how many weeks they would like to do the exercises for (example: 6 weeks, 18x6= 108 dollars). This is easy to do on excel but i cant seem to find a tutorial on how to do this on square space. Should i higher an expert?