Let p be the number of pennies and n be the number of nickels. As you have $8.80 in total (given that pennies are worth $0.01 and nickels are worth $0.05)
0.01p + 0.05n = 8.80
You also know that there are twice as many nickels than pennies (or alternatively, there are 2 nickels for every penny).
n = 2p
You can substitute this value of n into the first equation
0.01p + 0.05n = 8.80
0.01p + 0.05(2p) = 8.80
0.01p + 0.10p = 8.80 (multiplying out the brackets)
0.11p = 8.80 (collecting the p's)
p = 80
Putting this back into the second equation
n = 2p
n = 2(80) = 160
So there are 160 nickels and 80 pennies.
You can put this back into the first equation to check it.
0.01p + 0.05n = 8.80
0.01(80) + 0.05(160) = 0.80 + 8 =8.80
