Thread: Equation help

The teacher gave this problem. She gave the weight of an old penny as 3.09g and the weight of a new penny as 2.51g. There are ten pennies in a container and the combined weight is 26.78. She wants an equation to find how many of each penny is in the container. The equation created was 3.09X+2.51y=26.78. Was told that this is incorrect because cannot have two variables in this equation.
Help. Mom lost.

You're on the right track, you just need a 2nd equation because you do have two variables to solve for. The total amount of pennies is 10, so you can also say x+y=10.




Now you either use y=10-x or x=10-y to make a substitution into the 1st equation and solve for them both.

We were substituting the wrong set of numbers. There was a second container with pennies as well and I was trying to solve for x and substitute that set but it wasn't working. I guess it's been too long since I did algebra. Thanks so much for putting us on the right track.