Supposed you work summer house-sitting. Most of your customers pay you cash.
A few ask you to send them a bill. Is is now June 30 and you have collected $1000
From cash-paying customer.
Your remaining customers owe you $1500.
How much service revenue you have under a accural basis?

I have hard time understaind this problem. Please help me!

When do the customers pay you? If after you provided the service, then just add your cash received as well as what is owed to you.

In accrual basis of accounting you will record your fees when earned. In your problem you have earned the revenues because you have provided services and have either received cash or you will bill them for the services that you have already provided.