text: word problem solving with two unknowns, using rate(time)=distance? a baker purchased 12lbs of wheat flour and 15lbs of rye flour for a total cost of $18.30.  a second purchase, at the same prices, included 15 lbs of wheat flour and 10lbs of rye flour.  the cost of the second purchase was $16.75.  find the cost per pound of the wheat flour and of the rye flour. let w = price of wheat flour per pound let r = price of rye flour per pound  first we have to set up our system of equations. 12w + 15r = 18.30 15w + 10r = 16.75  now we will use substitution to solve for a variable.   12w + 15r = 18.30 subtract 12w from both sides. 15r = 18.30 - 12w divide both sides by 15. r = 1.22 - .8w  now we substitute r into the other equation. 15w + 10r = 16.75 15w + 10 (1.22 - .8w) = 16.75 15w + 12.2 - 8w = 16.75 7w + 12.2 = 16.75 7w = 4.55 w = .65  now we substitute w back into one of the equations to get r. 12w + 15r = 18.30 12(.65) + 15r = 18.3 7.8 + 15r = 18.3 15r = 10.5 r = .7  so, wheat flour costs $0.65 per pound and rye flour costs $0.70 per pound.  *********************************************************************  now we can check our answer in the other equation.  15w + 10r = 16.75 15(.65) + 10(.7) = 16.75 9.75 + 7 = 16.75 16.75 = 16.75  we are good! :)