Chlorination is used in industry for the purification of

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BTGmoderatorLU: Moderator; Posts: 2505; Joined: Sun Oct 15, 2017 1:50 pm; Followed by:6 members

Chlorination is used in industry for the purification of

by BTGmoderatorLU » Thu Jan 31, 2019 3:13 pm

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Source: GMAT Prep

Chlorination is used in industry for the purification of water; 1gm solution of chlorine will remove half of the germs in 1 liter of water. If water containing less than 0.09% of germs is considered clean, what is the minimum amount of chlorine required to purify 150 liters of water containing 0.3 liters of harmful substances?

A. 1
B. 2
C. 3
D. 4
E. 5

The OA is B

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Source: Beat The GMAT — Problem Solving | Thu Jan 31, 2019 3:13 pm

deloitte247: Legendary Member; Posts: 2214; Joined: Fri Mar 02, 2018 2:22 pm; Followed by:5 members

by deloitte247 » Sun Feb 03, 2019 8:27 am

Concentration of germs per harmful substances in 150 liters of
$$\frac{0.3}{100}\cdot100\%=0.2$$
Concentration of germs must be less than 0.09% for water to be considered safe
1 gm will reduce concentration of germs by half.
1 gm will reduce 0.2
$$\frac{1}{2}\cdot0.2=0.1$$

0.1% is still greater than 0.09% hence we will add another 1 gm to reduce to 0.1%
1 gm will reduce to 0.1
$$\frac{1}{2}\cdot0.1=0.005\%$$
0.05% is lesser than 0.09%, water is now safe

2 gm of chlorine was needed to purify the water.
$$answer\ is\ Option\ B$$

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Scott@TargetTestPrep: GMAT Instructor; Posts: 8086; Joined: Sat Apr 25, 2015 10:56 am; Location: Los Angeles, CA; Thanked: 43 times; Followed by:29 members

by Scott@TargetTestPrep » Mon Feb 04, 2019 5:23 pm

BTGmoderatorLU wrote:Source: GMAT Prep

Chlorination is used in industry for the purification of water; 1gm solution of chlorine will remove half of the germs in 1 liter of water. If water containing less than 0.09% of germs is considered clean, what is the minimum amount of chlorine required to purify 150 liters of water containing 0.3 liters of harmful substances?

A. 1
B. 2
C. 3
D. 4
E. 5

The water has 0.3 liters of harmful substances and needs to be reduced to less than 0.0009 x 150 = 0.135 liters in order to be considered clean.If we add 1 gram of chlorine to the solution, half of the germs will be removed and we will be left with 0.3 x 1/2 = 0.15 liters of harmful substances. If we add another gram of chlorine, there will be 0.15 x 1/2 = 0.075 liters of harmful substances remaining in the water, which is a low enough figure for the water to be considered clean. Thus, we need 2 grams of chlorine in total.

Answer: B

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