well on this type of question you can also logically guess...
we know that Mix X has 40% Ryegrass
we know the Mix Y has 25% Ryegrass.
now if we both X+Y are combined the resulting ryegrass is 30%
well we logically see that since resulting rye grass is less than what it was in Mix X, then Mix X has to be less than 50% ...however we can also see that the resulting MIX has has 30% Ryegrass which is more than Mix Y, then answer choice 10% can also be easily elminated...we are now left with 2 choices 33% or 40% (you are down to 50-50 chance of getting it right)
anywar here is how I would solve it
40X + (1-X)25=30
x=1/3 or 33.33%