Let p = number of pennies.
Let n = number of nickels.
We are given that n= 2p and the total value is $8.80.
We know that a penny = $0.01 and that a nickel = $0.05.
So $0.01p + $0.05n = $8.80.
Substitute 2p for n:
$0.01p + $0.05*2p = $8.80
$0.01p + $0.10p = $8.80
$0.11p = $8.80
p = 80
So n = 2p = 2*80 = 160
Thus there are 80 pennies ($0.8) and 160 nickels ($8.00). The value of all the coins is $8.80.