The unknown quantities are the unit cost of a tape and of a CD. Let x = the cost of a CD. Then (x - 4) is the cost of a tape. The total cost of the purchase is 70 dollars. Since there was one CD and five tapes involved in the purchase, the total cost can be written algebraically as:
x + 5(x - 4) = 70
Can you solve this problem for x, Lola?