Question 811464
Most of what you said for the problem is wrong
let f= number of $5 bills
let t= number of $20 bills
f+t=54,
5f+20t=780
f = 20,   t = 34
There are 20 $5 bills and 34 twenty dollar bills
check
f=20 and t=34
5*20=100
34*20=680
680+100=780
ok