Question 433560
You are making 10 tons of concrete that is
62% cement by mixing a 20% cement mixture
with a 80% cement mixture. 
a) How much of the 80% cement mixture
must you use?
..
let x=tons of 80% mixture to use
then 10-x tons of 20% mixture to use
80%x+20%(10-x)=62%(10)
.8x+2-.2x=6.2
.6x=4.2 
x=7 tons

ans:
7 tons of 80% mixture is mixed with 3 tons of the 20% mixture to get 10 tons of 62% concrete.