Directions: Study the following questions carefully and choose the right answer:
1
The ratio of zinc and copper in a brass pieces is 13 : 7. How much zinc will be there in 100 kg of such a piece?
» Explain it
D
Amount of zinc =  ( 100 ×  13 ) kg = 65kg.
20

Hence, option D is correct.
2
The ratio of number of men and women in a factory of 720 workers is 7 : 5. How many more women should be joined to make the ratio 1 : 1? 
» Explain it
C
Number of men =  ( 720 ×  7 )  = 420.
12

Number of women = (720 - 420) = 300.

∴   Number of women to be joined = (420 - 300) = 120.

Hence, option C is correct.

3
What is the ratio whose terms differ by 40 and the measure of which is 2/7 ?
» Explain it
B
Traditional Method:

Let the ratio be x : (x + 40).

Then,   x  =  2   ⇔ 7x = 2x + 80  ⇔ x =16.
(x + 40) 7

∴   Required ratio is 16 : 56.
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Smarter Approach1;

Pick one option and check whether it satisfies the given condition or not. The one that satisfies will be the answer.

Option A:  12  
56

gets eliminated because the difference between the terms is not 40.

Option B:  16  
56

is the correct answer because the difference between the terms is 40 and

its measure is also  2 .
7
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Smarter Approach 2:

Step 1:  Take the difference between the terms of given ratio 2/7.

                                                     7 ~ 2 = 5

Step 2: Divide the actual given difference between the term by the resultant we got in step 1.

  40  = 8
5

Step 3: Take this quotient as a multiple and multiply it into the numerator and the denominator part of the given ratio to get the actual ratio.

  2 × 8  =  16
7 × 8 56

Hence, option B is correct.

4
Two numbers A and B are such that the sum of 5% of A and 4% of B is two-third of the sum of 6% of A and 8% of B. Find the ratio of A : B is
» Explain it
A
From the equation, we get

5% of A + 4% of B =  2  (6% of A + 8% of B)
3

⇒ 15% of A + 12% of B = 12% of A + 16% of B

⇒ [15 – 12] % of A = [16 – 12] % of B

⇒ 3 % of A = 4% of B

⇒  A  =  4
B 3

Therefore, A : B = 4 : 3.

Hence, option A is correct.

5
In a certain school, the ratio of boys to girls is 7 : 5. If there are 2400 students in the school, then how many girls are there?
» Explain it
D
Let the number of boys and girls are 7x and 5x, respectively.

Given, total number of students = 2400

⇒ 7x + 5x = 2400  ⇒  12x = 2400 

   x = 200

∴    Required number of girls = 5x  =  5 × 200 = 1000.

Hence, option D is correct.