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.