K = 120 is the answer
Forgot to multiply 150..sorry
not able to solve..
Note in 2 step n=5 and not 21
bro isn't this method too lengthy to be done during exam? 2,3,7,73 are the prime factors..bahaut lengthy ho jaayega..
I know.But aur koi method hi dimag me nahi aa raha. Last year Jee advance me yehi level ke calculation puche the. So I think this is acceptable.
But please if anyone has alternative solution, please share your approach. @Sneha_2021 @akash_2020 @Achyut_2020
Put x=i in expansion of 21(1+x)^20 compare real part
Can you send this part in written
Couldn't upload,slow net
After comparing u will get,
21C1-3x21C3+5x21C5... = -21x2^10
- not exact problem but like @akash_2020 said