Directions: Study the following questions carefully and answer the questions given below.
1
The average age of A and B is 20 years. If A is to be replaced by C, the average would be 19 years. The average age of C and A is 21 years. The ages of A, B and C in order (in years) are
» Explain it
C
A + B = 2 × 20 = 40 yr        

B + C = 2 × 19 = 38 yr

C + A = 2 × 21 = 42 yr

On adding all three,

2 (A + B + C) = 40 + 38 + 42 = 120

⇒ A + B + C = 60

∴    A = (A + B + C) – (B + C) = 60 – 38 = 22 yr

Similarly,

B = (A + B) – A = 40 – 22 = 18 yr

C = (C + A) – A = 42 – 22 = 20 yr
 
Note: In this question we can save 4-5 seconds by not calculating the age of the third person as with only the respective ages of A and B we can confirm the correct answer out of the given options.

Hence, option C is correct.
 
2
The mean high temperature of the first four days of a week is 25°C where as the mean the last four days is 25.5°C. If the man of the whole week is 25.2°C then the temperature of the 4th days is
» Explain it
D
To solve this question we can apply a short trick approach

Value of  ( n + 1 ) th result =   ( n + 1 )  × (b + c) – n × a
2   2

Where

n is the total number of term = 7 days

b is the average of first four terms = 25

c is the average of last four terms = 25.5

a is the average of whole terms = 25.2

By the short trick approach, we get

Value of  ( n + 1 ) th result =  ( 7 + 1 )  × (25 + 25.5) – 7 × 25.2
2   2

( 8 )  × (50.5) – 176.4  ⇒  202 – 176.4 = 25.6  
2

Traditional method: 

Total average of first 4 days = 4 × 25 = 100

Total average of last 4 days = 4 × 25.5 = 102

Total average of 7 days = 7 × 25.2 = 176.4

Temperature 4th day's = 100 + 102 – 176.4 = 25.6

Hence, option D is corrrect.

3
A shop of electronic goods is closed on Monday. The average sales per day for remaining six days of a week is Rs. 15640 and the average sale of Tuesday to Saturday is Rs. 14124. The sales on Sunday is
» Explain it
B
Total sales of 6 days = 6 × 15640 = 93840
 
Total sales Tuesday to Saturday = 5 × 14124 = 70620
 
Sunday's sale = 93840 – 70620 = 23220.

Hence, option B is correct.
 
4
Find the average of the following set of numbers
 
354, 281, 623, 518, 447, 702, 876

» Explain it
D
Average =  354 + 281 + 623 + 518 + 447 + 702 + 876
7

3801  = 543
7

Hence, option D is correct.

5
The average marks obtained by a student in 6 subjects is 88. On subsequent verification it was found that the marks obtained by him in a subject was wrongly copied as 86 instead of 68. The correct average of the marks obtained by him is
» Explain it
C
Total number of marks = 88 × 6 = 528

Now, 528 – 86 + 68 = 510

Required average =  510  = 85.
6

Hence, option C is correct.