Solution:
Total age of 5 members = 5 × 28 = 140
And, required average
= $$\frac{140 - 20}{4}$$
= $$\frac{120}{4}$$
= 30 years

Solution:
Average cost of 5 apples and 4 mangoes = Rs. 36 Total cost = 36 × 9 = 324 Average cost of 7 apples and 8 mangoes = 48 Total cost = 48 × 15 = 720 Total cost of 12 apples and 12 mangoes = 324 + 720 = 1044 Therefore, cost of 24 apples and 24 mangoes = 1044 × 2 = 2088

Solution:
According to the question,
Average weight of the 12 crewman increased by = $$\frac{{1}}{3}$$ kg
∴ Total increase in weight
$$\eqalign{ = 12 \times \frac{1}{3} \cr = 4{\text{ kg}} \cr} $$
Weight of old man = 55 kg
Weight of new man = 55 + 4 = 59 kg

Solution:
Average temperature of first four days = 25°C
Total temperature of first four days = 25° × 4 = 100°C
Average temperature last four days = 25.5°C
Total temperature of four days = 25.5° × 4 = 102°C
Total temperature of whole week = 25.2° × 7 = 176.4°C
∴ Temperature of the 4th day
= 100° + 102° - 176.4°
= 25.6°C

Solution:
Let the number of boys and girls in the class are x and y.
According to given information,
⇒ 30x + 20y = 23.25 (x + y)
⇒ 30x + 20 y = 23.25x + 23.25y
⇒ 30x - 23.25x = 23.25y - 20y
⇒ 6.75x = 3.25y
⇒ $$\frac{x}{y}$$ = $$\frac{3.25}{6.75}$$
⇒ $$\frac{x}{y}$$ = $$\frac{13}{27}$$
Hence, possible number of boys and girls 13 and 27 respectively.

Solution:
$$\eqalign{ {\text{Required average}} \cr {\text{ = }} {\frac{{67 \times 2 + 35 \times 2 + 6 \times 3}}{{2 + 2 + 3}}} \cr = {\frac{{134 + 70 + 18}}{7}} \cr = \frac{{222}}{7} \cr = 31\frac{5}{7}{\text{years}} \cr} $$

Solution:
Average of first 'n' natural number
$$\eqalign{ = \frac{{\left( {n + 1} \right)}}{2} \cr = \frac{{93 + 1}}{2} \cr = 47 \cr} $$