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.
[tex]\bf \begin{cases}
f+t=133\implies \boxed{f}=133-t\\
5f+10t=705\\
-------------\\
5\left( \boxed{133-t} \right)+10t=705
\end{cases}
\\\\\\
665-5t+10t=705\implies 5t=40\implies t=\cfrac{40}{5}\implies t=8[/tex]
how many fives are there? well, f = 133 - t.
