How do you find 80 decreased by 65%

Since you're trying to find the percentage of something, you'd know that the original number (80) as a whole is 100%. In this problem, you're trying to decrease 65% of the 80. 80 is 100%. So, subtract 100% by 65%, and you're left with 35%. This 35% is the remainder of the 80 after 65% has been taken away. Now just multiply 80 by 35% (know your decimal places: 35% is 0.35), and you've got your answer.

Another way (probably the easier) to do this is to simply multiply 80 by 65% (0.65). Once you have that, just subtract it from 80.

After you've tried this problem by yourself, you should've gotten: 28.
Work:
80×0.35=28
OR
80÷0.65=52
80-52=28

Because the first wage is determined, here's the start of our sentence:
4(30)
Now, we know that any number above thirty turns into $5 an hour, so:
4(30) + 5(h - 30)
h represents hours, but because he only gets 5 an hour for his work above 30 hours, we must subtract five from the sentence.
So the function:
f(h) = 4(30) + 5(h - 30)
Or, simpler:
f(h) = 5h - 30