Question 1203240
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Hints:


a = number of 5's
b = number of 10's
c = number of 20's


a+b+c = 71 is one equation to form since there are 71 bills overall.


c = a+b-7 because of the sentence "The number of $20 bills is 7 less than the total number of $5 bills and $10 bills"


Another equation to form is 5a+10b+20c = 925 because there's $925 total in the cash register.
5a = amount of money from the 5's only
10b = amount of money from the 10's only
20c = amount of money from the 20's only


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So we have this system of equations
a+b+c = 71
c = a+b-7
5a+10b+20c = 925


There are 3 equations and 3 unknowns.


Try to make an equivalent system such that there are 2 equations and 2 unknowns.
Use the substitution property to replace every copy of 'c' in equation (1) with a+b-7. This gets rid of c and we're left with 'a's and 'b's only. Repeat the same idea for equation (3) as well.


I'll let the student take over from here.


Let me know if these hints help or not. 
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