which is what he did

[navi tasan]

1) He was hired to work ten hours a day to solve the problems we had, which is what he did.
2) He was hired to work ten hours a day and night to solve the problems we had, and that is what he did.

What do the parts after the comma modify?

I see two possibilities:
a. to work ten hours a day to solve the problems we had
or
b. to solve the problems we had

In most scenarios they boil down to the same thing, however there might be cases where they don't.

3) Tom says: Pete was hired to work ten hours a day, and he worked only seven hours a day.
James replies: Yes, but he was hired to work ten hours a day to solve the problems we had, which is what he did/and that is what he did. What difference does it make if he worked ten hours a day or not?

 

[emsr2d2]

Without further context, I would take 1 and 2 to mean that he both worked ten hours a day and solved the problems.
I'm not sure why 2 has "and night" added after "ten hours a day". That's certainly confusing because we don't know if he has to work ten hours spread over the daytime and the nighttime, or if he's meant to work ten hours during the day and another ten hours during the night, making a total of 20 hours' work.