At time 8:15, in the second method why the last 3 numbers are taken?? & in the 1st method only last digit is taken.
How can I understand, how much digit should I take from last??
Please note that they are two different methods. In the case of the first method, we choose the last digit then double it and then subtract it from the remaining number. For example, consider 483, 48 - (3*2) = 42, so 42 is divisible by 7, hence 483 is also divisible by 7. In the second method, we form an alternating block of 3 digits from right and add odd blocks from the right and subtract even blocks from the right. For example consider 1,369,851 divide into blocks of three digits from right and perform operations as said i.e, 851-369+1 = 483 = 7*69,So divisible. Actually, there are many more ways to check divisibility by 7, you can refer to them in this link: "https://en.wikipedia.org/wiki/Divisibility_rule#Divisibility_rules_for_numbers_1%E2%80%9330".
Learning any one of them will be sufficient.
In divisibility of 11, we always substract sum of odd - sum of even or it can be vice versa?
we can do (sum of odd terms - sum of even terms) or (sum of even terms - sum of odd terms) either way is correct..
22-(8+k)=0 => 22 = 8+k => k = 14
Thanks for pointing it out.
What about divisible of 12 ,13 , 14 , 15 etc whether we should do the same thing like 8 x 9 and check their divisibility or is there any other way around?
Please refer to the following link:"https://en.wikipedia.org/wiki/Divisibility_rule#Divisibility_rules_for_numbers_1%E2%80%9330".You will find easy methods to check divisibility of a number by divisors(1-30).