Question 5727
You can start by letting x = the # of lbs of the 9.3%-cement concrete and y = the # lbs of the 11.3%-cement concrete.

Next, you can write the necessary equations you'll need to solve the problem.

x + y = 4500 lbs of 10.8%-cement concrete.  Rewrite as: y = 4500 - x
9.3%x + 11.3%y = 10.8%(4500)

You can solve this system of equations by the "substitution" method. In the 2nd equation, you'll substitute y = 4500 - x and solve for x.  Change the percents to decimal.  9.3% = 0.093, 11.3% = 0.113, and 10.8% = 0.108

0.093x + 0.113(4500 - x) = 0.108(4500)  Simplify and solve for x.
0.093x + 508.5 - 0.113x = 468
-0.02x = -22.5
x = 1125 lbs.

y = 4500 - x
y = 4500 - 1125
y = 3375 lbs.

Finally:
1125 lbs of 9.3%-cement concrete were used.
3375 lbs of 11.3%-cement concrete were used.