A 20 lb. bag of economy brand cement mix contains 25% cement and 75% sand. How much pure cement much be added to produce a cement mix that is 40% cement?
2006-10-01 17:11:46 · 6 answers · asked by Denise L 1 in Science & Mathematics ➔ Mathematics
No it isn't due tomorrow. I'm terrible at math...I've tried and on this one I got that it had to be 1500% which just doesn't seem right...
2006-10-01 17:16:20 · update #1
Thank you very much! I really do appreciate it!
2006-10-01 17:23:31 · update #2
total = 20 lb
25% is cement = 5 lb
sand = 15 lb
now you want 40% cement
so sand should be 60%
if total is 100 then sand is 60
if sand is 60 total is 100
if sand is 15 total is 100/60*15 = 25
so cement = 10 lb
so you have to add cement = 5lb
check:
after adding cement = 10
total = 25
% cement = 40
2006-10-01 17:21:30 · answer #1 · answered by Mein Hoon Na 7 · 0⤊ 0⤋
The 20 lb. bag contains 20 X .25 = 5 lb. of cement and 20 X .75 lb. of sand.
If you add only cement, the resulting mix will still have 15 lb. of sand, but the sand will be only 100% - 40% = 60% of the mix.
Let w = the weight of the resulting mix. Then .60 X w = 15 lb., or
w = 15/.60 = 25 lb. Since you started with 20 lb., you need to add 5 lb. of cement.
2006-10-01 17:21:04 · answer #2 · answered by wild_turkey_willie 5 · 0⤊ 0⤋
Well this bag contans 25% cement and 75% sand
So cement content = 25% 0f 20 lb
= 5 lb
and sand content = 15 lb
In new mix 15 lb sand = 60% of mix
So total weight of mix = 15 lb / 60%
= 15 lb / 0.6
= 25 lb
Thus the weight must increase by 5 lb
Thus 5 lb of cement must be added to the mix
2006-10-01 17:20:47 · answer #3 · answered by Wal C 6 · 0⤊ 0⤋
the solution is to find the amount of weight that you have of cement and the amount of sand in the 20lb bag. Next you figure out how much cement would be needed with that amount of sand for the new mixture. After that you subtract the amount of cement you need from what is already there and what is left over is what you should add.
Now in numbers
cement = (.25)*(20lb) = 5lb
sand = (1 - .25)*(20lb) = 15 lb
(new mixture total)*(.6) = 15lb divide both sides by .4
new mixture total = 25
new cement total = new mixture total - 15 = 10
cement to add = new cement total - 5 = 10-5 = 5lb
2006-10-01 17:24:34 · answer #4 · answered by Joel D 2 · 0⤊ 0⤋
Let pounds of cement extra = x
So, (x+0.25*20)/(20+x) = 40/100
or 100(x+5) = 40(20+x)
or 60x = 7500
or x = 125
Thus 125 lbs of cement has to be added.
2006-10-01 17:26:20 · answer #5 · answered by ag_iitkgp 7 · 0⤊ 0⤋
Do your own homework.. Its Sunday night, is it due tomorrow mornining or something?
2006-10-01 17:13:19 · answer #6 · answered by Guru 6 · 0⤊ 1⤋