sally has twice as many dimes as nickels. The total value is $3.50. How many nickels dow she have?
Let's let x represent the number of nickels Sally has.
Since Sally has twice as many dimes as nickels, she has 2x dimes.
The value of a nickel is $0.05, so the value of the nickels is 0.05x.
The value of a dime is $0.10, so the value of the dimes is 0.10(2x)=0.20x.
The total value is 0.05x+0.20x=0.25x.
Since the total value is $3.50, we can set up the equation 0.25x=$3.50.
We can solve this equation by dividing both sides by 0.25: x=$3.50/0.25=14
Therefore, Sally has 14 nickels. Answer: \boxed{14}.