Question 1172204
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Tutor @josgarithmetic likes that formula with all those variables for solving mixture problems like this.<br>
If you use that method to solve mixture problems, you won't learn anything about problem solving.  Plugging numbers into a formula doesn't really teach you anything.<br>
Tutor @ikleyn shows a standard algebraic method for solving the problem.  If you pay attention to how the method works, you will learn something useful about problem solving using formal algebra.<br>
But if a quick and easy solution without formal algebra is acceptable, here is the fastest way to solve this, or any similar mixture problem.<br>
(1) The price per kg of the mixture $0,13, is 3/5 of the way from $0.10 to $0.15.  (If you don't see that, picture the three numbers 0.10, 0.13, and 0.15 on a number line....)
(2) That means 3/5 of the mixture is the more expensive ingredient.<br>
ANSWER: 3/5 of 300kg, or 180kg, of the more expensive granite chips; the other 120kg is the common stone.<br>
CHECK:
180(0.15)+120(0.10) 27+12 = 39
300(0.13) = 39<br>