In one mixture equation there is .06 + 2(.15)=.12(x + 2).

In another there is .03x + (900-x) = .08(900)

then on http://www.purplemath.com/modules/mixture.htm you get

If A + C = 548, then A = 548 – C (or C = 548 – A;

and 0.10(10 – y) + 0.30y = 1.5

then on http://www.purplemath.com/modules/mixture2.htm

Two hundred liters of a punch that contains 35% fruit juice is mixed with 300 liters (L) of another punch. The resulting fruit punch is 20% fruit juice. Find the percent of fruit juice in the 300 liters of punch.

There is no mention of adding or subtracting. Is Mixed either subtraction or addition?

Someone wants to mix milk cantaining 2% butter with cream containing 25% butterfat to obtain 1000 gallons of milk which is 8% butterfat. How much of each must he use?

In the milk word problem there is no key words saying add or subtract. Yet the solution equation is .02x + .25(1000-x)=.08(1000). ("1000" minus "x" ?)

How do you know whether to Add or subtract in a mixture word problem?