So, wait. He starts off with $40 for working 3 hours.

That means that he earns $13.34 per hour. (It's a repeating decimal, so I rounded it)

If he works another 7 hours, then he earned $93.34 (It's also repeating).

But he spent 1/3 of that money earned in the 7 hours, so he spent $31.11.

That means he had around $62.22 left from those 7 hours.

You would then add the $40 with the $62.22 and would get:

$102.22

You understand?

(It's an estimate, because of the repeating decimals)