Question 1862
Let d = number of Douglas fir
Let p = number of pines

we know that d + p = 850 --eqn1
also, 300d + 225p = 217500 --eqn2

Re-arrange eqn1 for d, d=850-p and then put this into eqn2, so 300(850-p) + 225p = 217500.

Multiply out the bracket, gives 255000-300p + 225p = 217500. Collect terms together, -75p = -37500. Hence p=500 and from eqn1, d=350.

cheers
Jon.