The average age of 8 children of a family is 12 yr. If the age of 7 children are 12, 8, 14, 11, 9, 13 and 15 yr. then the age of 8th child will be?

Required ratio = (36 - 8)/(24 - 8) = 28/16 = 7 : 4

Length of the train = 280 m
Length of the platform = 280 x 3 = 840 m
Taken time = 6 min 40 s = (360 + 40) s = 400 s
? Required speed = Distance / Time = (840 + 280)/400
= 1120/400 = 2.8 m/s

Let my present age be x years.
? x - 20 = x / 3
? 3x - 60 = x
? 2x = 60
? My present age = x = 30 years

Let B;s age 2 years ago be N years
? A's present age = 2N years

From question, 2N - (N + 2) = 2
? N = 4
? A's age = 2 x 4 = 8 years

Let the age of A be (x - 3) years
? B's age = x years
? C's age = (x - 3) + 2 = x - 1
? B's age - C's age = x - (x - 1) = 1 year
? B is one year older then C.

Let the ages of A and B are 4N and 5N yr. respectively.
According to the question,
? (4N + 4)/(5N + 4) = 14/17
? 68N + 68 = 70N + 56
? 2N = 12
? N = 6
Hence present age of B = 5 x 6 = 30 yr

Let present ages of indira and Lizzy are 3k yr and 8k yr, respectively.
? 3k + 8 = 20 ? 3k = 12
? k = 4
Hence, Lizzy's required age = 8k - 5
= 32 - 5
= 27 yr

Let 7 yr ago, ages of A and B 4k yr and 5k yr, respectively.
Then, present age of A = 4k + 7
and present age of B = 5k + 7
Now, according to the question,
? (4k + 7 + 7) / (5k + 7 + 7) = 5/6
? 24k + 84 = 25k + 70
? k = 14
Hence, B's present age = 5 x 14 + 7 = 77 yr

Let Sarita;s present age = k yr
Then, Kavita's present age = 2k yr
According to the question,
(2k + 8) / (k + 8) = 22/13
? 26k + 104 = 22k + 176
? 4k = 72
? k = 18
Hence, Kavita's age = 2 x 18 = 36 yr

According to the question,
(36 + n) / (50 + n) = 3/4
? 144 + 4n = 150 + 3n
? n = 6