Taylor's nickels and dimes total $7.60. If the nickels were replaced by pennies then she would have $6.80. How many of each coin does she have?
using the value of the coins (in cents), we have
5n+10d = 760
1n+10d = 680
subtract and you have
4n = 80
n = 20
so, d = 66
Or, you can notice that he loses 4 cents for each nickel-penny swap.
Since he lost 80 cents, that'd be 20 nickels changed to pennies.