BPP Kit

Hello Sir,
I was working on extension of credit and the below question is from BPP study kit:
Enticement Co currently expects sales of $50,000 a month. Variable costs of sales are $40,000 a month
(all payable in the month of sale). It is estimated that if the credit period allowed to accounts receivable
were to be increased from 30 days to 60 days, sales volume would increase by 20%. All customers would
be expected to take advantage of the extended credit. If the cost of capital is 12.5% a year, is the extension
of the credit period justifiable in financial terms?
My question : How did they calculated Accounts receivable after implementing the proposal (2 months) – 120,000
My calculation : 60*60,000/360 = 10,000
Isnt it should be calculated like in the above example of the page in the book as in that they calculate it for Average inventory and subtracted it with current avg inventory , why there is a different approach followed here ?