The class treasury contained 30$ in nickels and dimes. if there were 500 coins than how many coins of each type are there?

Equation 1.  N+D=500

equation 2   5N+10D=3000
This is for equation 1
N=500-d
Substitute into equations 2 and solve for D
5N+10D=3000
5(500-D)+10D=3000
2500-5D+10D=3000
5D=500
D=100
Lets go back the equation 1
N=500-D
N=500-100
N=400
there are 400 Nickles and 100 Dimes

The second and fourth answer are correct