I already know the answer just need an explanation on how they got $10,500

Question

March sales accounted for 1/7 of the total number of jazz records sold all year. About how many jazz records were sold all year? March sales was $1500

Answer 1

Note that there is sufficient information to answer the total dollars of the sales, but there is not sufficient information to answer how many records were sold.

x = total sales of the entire year

(1/7)x = March sales = 1500

x/7 = 1500

x = 7(1500) = 10500