Answer:
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
Explanation: