Question 1125271
2 equations - 

N = number of nickels.

Since total coins are 20,  N+D = 20  (D is number of Dimes)

5N is the value of the nickels, in total cents.
10D is the dimes total value in cents. 

5N+10D = 125

and 

N+D = 20

2 equations, 2 unknowns. 

You can also use 20-N as the number of dimes, and 10(20-N) as their value, to produce one equation in one unknown.