Calvin has $8.80 in pennies and nickels. if there are twice as many nickels as pennies, how many pennies and nickels does calvin have?

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