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| 1. |
\begin{array}{l}{\text { Find } n \text { and } r, \text { if }} \\ {\text { (a) }^{n} P_{r}=720 \text { and }^{n} C_{r}=120} \\ {\text { (b) }^{n} C_{r-1} :^{n} C_{r} :^{n} C_{r+1}=20 : 35 : 42}\end{array} |
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