SOLUTION: A jar contains only nickels and dimes and there is 200 coins in the jar. The value is 14 dollars. How many nickels and dimes are in the jar?

Let n = # of nickels and d = # of dimes.


Then your equations are 

 n +   d =  200,   (1)
5n + 10d = 1400.   (2)

To solve this system, express n via d from (1): n = 200 - d, and substitute it into (2). You will get

5*(200-d) + 10d = 1400.

Simplify it:

1000 - 5d + 10d = 1400,

5d = 1400 - 1000 = 400,

d =  = 80.

Answer. 80 dimes and 200-80 = 120 nickels.