In algebra you can use function to express the idea that salary depends on the number of hours of training?

Yes! You can express this as a function, like this:

In my example I will say that the salary is $240.00 and this is based on a standard 30 hours of training.

g(x)=30x +240

What we are basically saying is that whatever number we plug in for g(x) would determine how much they are paid. For example if we add 20 times the amount of hours of training we would get g(20)=30x + 240

Then plug in 20 for our x 30(20) +240 the new salary would be $840

:)