The metric system units are based on the powers of 10.
Answer: Ratio of Parts
<u>Step-by-step explanation:</u>

The ratios are equal so the triangles are similar.
They can buy 6 workbooks with $90. To find the unit rate we simply divide the cost ($60) by the number of books (4). That gives us $15, the cost of 1 workbook. So now to find out how many workbooks can be purchased with $90, we divide 90 by 15. That’s how I got 6 workbooks. Hope this helps.