SAT - Heart of Algebra problem

   Devin is a landscaper who needs to prepare different types of grass seed for his customers' yards. Bluegrass seed costs $2.00 per pound while drought-resistant seed costs $3.00 per pound. If for a particular day the two types of grass seed totaled $68.00 and together weighed 25 pounds, how many pounds of bluegrass seed did Devin prepare?

  a. 4

  b. 7

  c. 18

  d. 21

  e. 25