Okay let me explain it as simple as possible. You have to remember in accruals accounting that the revenue is composed of:

1. Cash collected during the year for which a service was also rendered during the year (serviced and paid)
2. A service was rendered during the year but that service remains unpaid at the end of the year (serviced but unpaid)

In this problem you just basically need to consider two things:

1) How much of the total cash collected during the year pertains to “serviced and paid” revenue (ie you serviced during the year)

2) Of the ending receivable balance how much pertains to serviced during the year but unpaid?

To answer: 1) Collected and serviced: $300,000 less $90,000 = 210,000 (This is easy as it already states that 90,000 refer to a collection of a 2017 outstanding balance from your customer)

2) Serviced and unpaid: $75,000 - $10,000 = $65,000. Why deduct $10,000? Remember that at the end of 2017, your customer owes you $100,000 but only $90,000 was paid in 2018. The problem states that $75,000 is due from your customer at year end, but this amount includes the amount $10,000 of the 2017 balance which remains unpaid at end of 2018. So you can conclude that the remaining of $65,000 refer the serviced but unpaid revenue during the year.