<PRE><B>Question 1207766</B>
&lt;br&gt;
The 20-pound bag is 25% (=1/4) cement and 75% (=3/4) sand, so it contains 5 pounds of cement and 15 pounds of sand.&lt;br&gt;
Let x be the number of pounds of cement you add.  Then the number of pounds of cement is 5+x, and the total weight is 20+x.&lt;br&gt;
You want the cement to be 40% of the total weight:&lt;br&gt;
(5+x) = 0.40(20+x)&lt;br&gt;
5+x = 8+0.40x
x-0.40x = 8-5
0.60x = 3
x = 3/0.60 = 5&lt;br&gt;
ANSWER: 5 pounds&lt;br&gt;
CHECK: the total weight is now 20+5 = 25 pounds; the number of pounds of cement is 5+5 = 10; 10/25 = 0.40&lt;br&gt;
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