The main thing to remember is that if the count of two objects add up to a total of X, then the amount of one object is N and the second object is X-N. Then you have enough info to setup the equation. I will choose to use N to represent Nickels, and 14-N to represent Dimes. If there are for example 2 Nickels, then there must be 14-2 = 12 Dimes.
A dime is 10 cents and a Nickel is 5 cents - this is information that you have to know that is not given in the question. It doesn't matter if you use .10 or 10 as long as you do it for all parts of the problem. If your answer had required dollars and cents, then you might have to move the decimal place at the end if you didn't do it in the problem itself.
So the final equation is 5N + 10(14-N) = 80. From there, just do the algebraic steps you would normally do to solve.
