Today, Reza ate 20 grapes, which was 400% as many grapes as he ate yesterday. Explain how to use equivalent ratios to find the number of grapes Reza ate yesterday.

What you must do for this case is to extract the relationship of the grapes that were eaten yesterday among the grapes that were eaten today.
 To do it, let:
 x: number of grapes that Reza ate yesterday
 (x / 20) = (100/400).
 Clearing x we have:
 x = (100/400) * (20) = 5
 answer
 the number of grapes Reza ate yesterday was 5