F = amount of five dollar bills
t = amount of ten dollar bills
so, he has a total of 133, thus whatever "f" and "t" are, we know that
f + t = 133.
how much money is there in "f" amount of fives? well, 5*f, or 5f.
how much money is there in "t" amount of tens? well 10*t or 10t.
we know the total amount is 705, therefore
5f + 10t = 705.

how many fives are there? well, f = 133 - t.