The value of \( \sum_{r=0}^{20} r(20-r)\left({ }^{20} C_{r}\right)^{2} \) is equal to (a) \( 400...

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The value of \( \sum_{r=0}^{20} r(20-r)\left({ }^{20} C_{r}\right)^{2} \) is equal to
(a) \( 400^{37} C_{20} \)
(b) \( 400{ }^{40} C_{19} \)
(c) \( 400^{38} C_{19} \)
(d) \( 400{ }^{38} C_{20} \)
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