Important for :

1

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

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

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.

Hence, option C is correct.

2

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 |

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

4

5