To start, what do you know?...you know that you have:
2x as many nickels than quarters
and
the total of the quarters and the nickels will give you $2.80.
So if X = the number of quarters, then 2X = the number of nickels.
Since you need to take into account the value of each coin, your equation is:
2X(.05 for the amount of each nickel)+ X(.25 for the amount of each quarter)=2.80
Now simplify and solve:
2X(.05)+.25X = 2.8
.1X + .25X = 2.8
.35X = 2.8
.35X/.35 = 2.8/.35
X = 8
Since X = the number of quarters, you have 8 quarters.
Since 2X = the number of nickels, you have 16 nickels.
Check your answer:
8 quarters + 16 nickels should give you $2.80
8(.25) + 16(.05) =
2.0 + .8 = 2.8 or $2.80