With Roman numbers addition and subtraction was OK, division by 2 was OK too, but multiplication was very hard. Unfortunately, at the market one needs to multiply the price of items by the number of...

With Roman numbers addition and subtraction was OK, division by 2 was OK too, but multiplication was very hard. Unfortunately, at the market one needs to multiply the price of items by the number of items sold. Installing large multi-plication tables at the market would have been impractical. Instead, the ancient Romans installed large tables of square numbers. Why did this solve the problem?