Question 91966
the idea is that the amount from nickels plus the amount from dimes will the the total money, that's where comes the equation.

Let N be # of nickels
Let D be # of dimes

we have
 N + D = 50
5N + 10D = 350

solve this 2 by 2 system:

multiply the first equation by -5

-5N - 5D = -250

add to second equation, we get

5D = 100

D = 20

N = 50 - D 
= 50 -20 = 30

so the answer is there are 30 nickels and 20 dimes.