Question 67475
Let x  = the number of nickels she has and y = the number of dimes she has.  From the information provided, we can see that:
$.05(x) + $.10(y) = $3.50 and 
x + y = 50.  

From the second of these equations we can solve for x
x + y = 50 ==> x = 50 - y (by subtracting y from both sides)
Now we can plug this value of x back into the first equation above.
.05(50-y) + .10y = 3.50 (multiply the .05 over 50 - y)
[.05(50) - .05(y)] + .10y = 3.50
2.5 - .05y + .10y = 3.50 (add the y terms)
2.5 + .05y = 3.50 (now subtract both sides by 2.5)
.05y = 3.50 - 2.50
.05y = 1 (divide both sides by .05)
y = 20 dimes!

Now we can use the second of the original equations to solve for x!

x + y = 50
x + 20 = 50 
x = 50 - 20
x = 30.
So we have 30 nickels!

Lets check!
$0.05(30) + $0.10(20) = $1.50 + $2.00 = $3.50

There are 30 nickels and 20 dimes.

Hope this helps you! Hang in there and don't get discouraged!