Jack has twice as many dimes as quarters. If the total value of the coins is $6.30, how many dimes does he have?
Its 10.5 dimes, since you make two equations, 2x = y and 0.1x+0.25y = 6.30.
IF you solve it using systems of equations you get 10.5 dimes.\
Very unrealistic, but there are 10.5 dimes.