2m^3 of soil containing 35% sand mixed into 6m^3 of soil containing 15% sand. What is the sand content of the mixture?

You could use that formula to help you:

Amount = Concentration x Volume

So, find the amount of sand added from the 2 m^3 35% soil,
find the amount of sand added from the 6 m^3 15% soil,
add both amounts,
find the total volume that you get,
divide the total amount of sand by the total volume.

Can you post what you get? :)