Receivable management

Hello Sir, Can you please explain me how did they calculated the contribution/sales ratio in the below question of BPP Study kit: Grabbit Quick Co achieves current annual sales of $1,800,000. The cost of sales is 80% of this amount, but bad debts average 1% of total sales, and the annual profit is as follows. $ Sales 1,800,000 Less cost of sales 1,440,000 360,000 Less bad debts 18,000 Profit 342,000 The current debt collection period is one month, and the management consider that, if credit terms were eased (Option A), the effects would be as follows. Present policy Option A Additional sales (%) – 25% Average collection period 1 month 2 months Bad debts (% of sales) 1% 3% The company requires a 20% return on its investments. The costs of sales are 75% variable and 25% fixed. Assume there would be no increase in fixed costs from the extra revenue and that there would be no increase in average inventories or accounts payable. Which is the preferable policy, Option A or the present one?