Question 1083425
Let x = number of 10% bags of cement needed
The goal is to find the number that replaces x


Let's say that each bag weighs 100 pounds.


A single 10% cement bag has 10 pounds of pure cement (0.1*100 = 10)
x of these bags would have 10x pounds of pure cement
In total, we have 100*x = 100x pounds of material (cement+other stuff)


A single 12.5% bag has 12.5 pounds of pure cement (0.125*100 = 12.5)
Four of these bags would have 4*12.5 = 50 pounds of pure cement
In total, we have 4*100 = 400 pounds of material (cement+other stuff)


Mixing the x 10% bags and the four 12.5% bags yields 
<font color=blue>10x+50 pounds</font>
of pure cement


This would be out of <font color=green>100x+400 pounds</font> total (of cement plus other material)


Divide the two expressions (10x+50 and 100x+400) and set this ratio equal to the desired percentage we want, which is 11% = 0.11, so
(<font color=blue>amount of pure cement</font>)/(<font color=green>amount total</font>) = percentage desired
(<font color=blue>10x+50</font>)/(<font color=green>100x+400</font>) = 0.11


Solve for x
(10x+50)/(100x+400) = 0.11
[(10x+50)/(100x+400)]*(100x+400) = 0.11*(100x+400)
10x+50 = 0.11*(100x)+0.11*(400)
10x+50 = 11x+44
10x+50-44 = 11x+44-44
10x+6 = 11x
10x+6-10x = 11x-10x
6 = x
x = 6


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Answer: 


We need <font color=red>6 bags</font> of the 10% mix.