Question 297822
so let's first make the equations    x=dimes     y=quarters

There are x number of dimes and y number of quarters and together the make up 20 coins.

so  x+y=20  and if dimes are .10 and quarters are .25 then you can say
.10x + .25y = 3.80

Now you have the two equations
1) x + y = 20
2) .10x + .25y = 3.80
Let's use substitution and use equation 1, which we can rearrange to say
x = 20-y  now sub that into x in equation 2

.10(20-y)+ .25y = 3.80
2.0 - .10y +.25y = 3.80
2.0 + .15y = 3.80  subtract 2.0 from both sides
.15y = 1.80  divide by .15
y = 12

so there are 12 quarters...use y = y back into equation 1
x + 12 = 20  subtract 12 from both sides
x = 8

so there are 8 dimes and 12 quarters