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- Try to express numbers in base $7$.
- You can think of it in terms of expansion of (1+x). Then apply combinatorial logic in terms of the power of $x$.
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What will be the reminder when ${89}\choose{78}$ is divided by $7$?
[Here, $\binom{89}{78}={}^{89}C_{78}$]
Combinatorics Divisibility
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