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Question

Answer

(a) Difference in the stamps = 4+5 = 9
Find the pair of multiples of 7 and 8 with a difference of 9.
8-7= 1, 16-14 = 2, 24-21=3…..72-63=9
Hence there are 9 boys.
(b) 72 stamps in excess of 4 stamps => 72-4 = 68
63 stamps short of 5 stamps => 63+5 = 68
Hence number of stamps is 68


Multiples question.
Video Solution can be viewed here https://youtu.be/8onLBo-peKE
waterflow
(a)
If there are L boys ,
Then total stamps = 8L-4 = 7L+5
L=9
(b)
So there are 7*9+5=68 stamps.

Give the boys 7 stamps each. There are 5 extra left over. Give each of the 5 extra to the first 5 boys, so they have 8 stamps each. The question says we are now short of 4 stamps to make everyone own 8 stamps. That means 4 boys were not given their 8-th stamps. a. Number of boys = 5 (those who got the 8-th stamps) + 4 (those who didn't) = 9 b. There were 9 x 7 + 5 = 68 stamps. CHECK: 68 = 9 x 7 + 5 = 9 x 8 - 4 1234567 8 XXXXXXX . XXXXXXX . XXXXXXX . XXXXXXX . XXXXXXX X XXXXXXX X XXXXXXX X XXXXXXX X XXXXXXX X Because 7 stamps and 8 stamps differ by 1, we can use this simple 'story' to figure it out. :-)

Let there be N boys. Number of stamps is 8N - 4 (short of 4 if each have 8) It is also 7N + 5 (extra 5 if each have 7) So 8N - 4 = 7N + 5. Thus N = 9. There are 9 boys. Not sure if P4 does algebra though.