By mass a certain material is 75% sand and another is 30% sand. How many tonnes of each must be mixed in order to have 250 tonnes of material that is 50% sand?
75% amount --- x
30% amount --- 250-x
.75x + .3(250-x) = .5(250)
solve for x
30% amount --- 250-x
.75x + .3(250-x) = .5(250)
solve for x