well lets see...If we did the math about the total ammount of bills we will make a formula starting off as 5(124-x)-10x=840 and using the 5 dollar bills equal to x
distribute the 5 first for 620-5X-10X=840 840 minus 620 gives us 220 and divide 220 by 5 which will give us 44 5 dollar bills, now we do the 10 dollar bills. let Y=10 for the next equation of 5Y+10Y=840, substitute the 10Y with your answer for 5 resulting in 5Y+10(44)=840 10 times 44 is 440 and now subtract 440 from 840 resulting in 5Y=400 divide the 400 by 5 and get a new answer of Y equals 80 for ten dollar bills
X=44
Y=80
44+80=124
