algebra
posted by Jay .
Al has 76 nickels and dimes worth$5.55. How many of each coins does Al have?

Let x = # nickels and y = # dimes
x = 76  y
5x + 10y = 555
Substitute 76y for x in second equation and solve for y. Insert that value into the first equation and solve for x. Check by inserting both values into the second equation. 
Agree with this solution.
Worth pointing out that x = 76  y originates from the formula x + y = 76. In case Jay / anyone else can't seem to get that far when solving similar problems on their own.
5x + 10y = 555 because a nickel is worth 5 cents and dime is worth 10, for a total of 555 cents. When doing these in school I would sometimes assign a variable like n and d so I'd never forget what a variable represents or accidentally get them switched around in my answer.
This also carries over nicely when solving problems like these in computer programming.
Good stuff PsyDAG