Hello from MrBillDoesMath!
Answer: 3 days, 54 belts and wallets sold
Discussion:
After x days, belts sold = 3^x
Notice that "x" being days is probably an integer (A guess on my part)
After x days, wallets sold is 4x + 15.
Let's create a table and see if we can find the answer:
x 3^x 4x + 15
------------------------------------------------
1 3 4(1) + 15 = 19
2 9 (3^2) 4(2) + 15 = 23
3 27 (3^3) 4(3) + 15 = 12 + 15 = 27
So after day 3 the number of belts and wallets sold are the same. 27 units of each has been sold so the total number of sold items = 27 + 27 = 54
Regards, MrB