We have two different soils with percentages of sand that make them different. We want to mix these 2 and come up with what percentage of sand the mixture is. If we have 2 m^3 of one sand content and 6 m^3 of another sand content and we are mixing those, we will end up with 8 m^3 of the mixture. The first soil is 35% sand, or .35; the second soil is 15% sand, or .15. The table for this problem looks like this:
number of meters x % sand = total % sand
35% sand 2 .35 .7
15% sand 6 .15 .9
What we do with these columns, the first one and the last one, is add them. 2 + 6 is 8 meters of mixture; .7 + .9 is the total of sand percentage. What we don't know the percentage of sand in the new mixture, that's what we are looking for, so that is our x. We multiply the first column by the second column and set that equal to the third column, so 8 * x = 1.6. Dividing by 8 we get that x = .2. As a percentage, that's 20%. The new mixture is 20% sand.