A firm has projected sales in May, June, and July of $$100,$ $$200,$ and $$300,$ respectively. The firm makes $20$ percent of sales for cash and collects the balance one month following the sale. The firm's total cash receipts in July is$?$

In all the answers to this question on Chegg they show you are $*$ cash by $80\%$ what i dont under stand is where the $80\%$ is comeing from can you explain in detail.