Keisha has a collection of dimes and nickels worth a total of $4. if she has 50 coins in all, how many nickels does keisha have?

Nickels (n): $0.05
Dimes (d): $0.10

Set up a system of equations
Cost/Worth:  .05n + .10d = 4.00
Quantity:           n  +     d = 50

Use elimination or substitution to solve.
I am using the elimination method:

100(.05n + .10d = 4.00)  →    5n + 10d = 400
 -10(     n  +     d = 50    ) → -10n  - 10d = -500
                                              -5n           = -100
                                                 n            = 20

Plug that into one of the original equations to solve for "d"
   n +  d = 50
(20) + d = 50
          d  = 30

there are 20 nickels and 30 dimes.