20
Just count the nines in the numbers: 9, 19, 29, 39, 49, 59, 69, 79, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99.
Note: 99 contains TWO nines!

Alan had a red boat and a green boat.
Brian had a green boat and a blue boat.
Charles had a yellow boat and a red boat.
Darren had a blue boat and a yellow boat

No Probs, He sleeps at night.

‘2’

This single sequence in fact consists of two sequences: the first sequence is 2-3-4-5-… and the second sequence is 9-18-36-72-… . To make the puzzle harder, all digits have been placed apart. Conclusion: the next term in the sequence is a ‘2’.

The second statement

The digits of the requested two numbers obviously form descending sequences. Furthermore, if you have two pairs of numbers with equal sums, the pair of which the numbers have the smallest absolute difference, is the one of which the numbers have the largest product. Using this knowledge, the two numbers can easily be constructed by placing the digits one by one, starting with 9 and ending with 1:
9

8 -> 96

87 -> 964

875 -> 9642

8753 -> 9642

87531
Conclusion: the requested two numbers are 9642 and 87531 (and the product of these two numbers is 843973902).

No time at all it is already built

Let the sum of nine member (total) =x
and the age of old one=z
so its average 4 yrs before=(x+z)/10.
after 4 yrs let z be replaced by y.
so now avg=(x+4*10+y)/10


now (x+z)/10=(x+40+y)/10
so after solving it found
z=y+40.
so old person is 40yrs older than young one.

167161
664711
162166
164611
——— +
1158649

Stop Imagining