A stack of nickels and dimes is worth 2.55. Find the number of nickels if the number of dimes is 12 less than twice the number of nickels.?

Let the number of nickels be ( n ) and the number of dimes be ( d ). According to the problem, we have ( d = 2n - 12 ) and the total value equation ( 0.05n + 0.10d = 2.55 ). Substituting for ( d ) gives ( 0.05n + 0.10(2n - 12) = 2.55 ). Solving this, we find that ( n = 18 ), so there are 18 nickels.