Question 833512
Let's the number of $5 bills be "x" and the number of 1$ bills be "y"
Now we have the total is 14 bills, that means {{{x + y = 14}}}
Then we have the total money is $30, that means we have {{{5x + 1y = 30}}} (because x is $5 and y is $1) 
Now let's solve the system of equation :) 
*[invoke linear_substitution "x", "y", 1, 1, 14, 5, 1, 30 ]
So Joe has 4 $5 bills and 10 $1 bills 
TA-DAH
:D