Luke has 21 coins worth $3.90. If he has only dime and quarters, how many of each does he have?

Question

So what  I have gotten for my system of equations,is

 

.10x+.25y= $3.90

 

x=y+21

 

 I believe i am wrong with these equations.

 

Update: so these equations seem to be right, but the answer it's giving is isn't.

 There's my process i'v been doing

 

input the second equation into x

 

.10(y+21)+.25y=3.90

 

.10y+2.1+.25y=3.90

       -2.1           -2.1

 

.10y+.25y=1.8

 

.35y= 1.8

 

and thats where it got confusing so i just stuck on this part.

 

Answer 1

Based on the first equation, I can see that x represents dimes and y represents quarters.  This equation is correct because it shows the total cost.

 

The second equation should be      x+y=21         because the total number of coins is 21.

 

Then the second equation can rewritten as       x = 21 - y         for substituting into the first equation.