Aptitude - Numbers - Discussion

it can also be divided by 48 i.e (a^n-b^n is always divisible by a-b)

Then why not 50?

9 power odd number results in 9 at units place.
9 power even number results in 1 at units place.

In 49 or 9 power 15.... 15 is odd therefore 9 will come at units place.
9-1 = 8 which is divisible by option[A] i.e., 8

Hi guys,

Just remember these formula's and verified it yourself.

a^n-b^n is divisible by a-b for all n.
a^n-b^n is divisible by a+b if n is Even.
a^n+b^n is divisible by a+b if n is Odd.

In our questions 49^15-1 is divisible by 48 as per formula but option is not available. So we have to further drill down to get the answer. Further simplification it became 7^30-1.

From the above formula answer would be 6, 8 and 6 is not available in the options. So answer is 8.

@Prasanna.

48 is not possible because rules tell that if x power n subtract a power n then result is x power n please a power. So I can say that may be the 50 answer is also possible.

Agree @Rashid.

Can anyone please explain how to find 7 power 6-4 power 6 is divisible by?

X^n-y^n=x-y.

So,
49^15-1^15=49-1=48.
48÷8....48 is divisible by 8 so, 8 is answer.

49^15 -1
with the help of cyclicity table the unit digit will be 9.

Unit digit for 49^15 = 9.

Now 9 -1 (given in question) = 8 answer.

Anyone, please explain me to get it.