Doubt from Binomial expansion

Mera answer k=0.8 aa rha hai..pls cross check someone..

K = 120 is the answer

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Solution pls

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Forgot to multiply 150..sorry :sweat_smile:


not able to solve..

I solved this and getting 2(2 and 3) but I dont know how to check for. each prime mumber

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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..:sweat_smile:

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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

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Put x=i in expansion of 21(1+x)^20 compare real part

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Can you send this part in written

Couldn't upload,slow net

After comparing u will get,
21C1-3x21C3+5x21C5... = -21x2^10

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