1.

nP5=42(nP3), find n

Answer» `nC_5*5! =42*nC_3*3!`
`(n!)/(5!(n-5)!)*5! =42*(n!)/(3!(n-3)!*3!`
`((n-3)!)/((n-5)!)=42`
`n^2-7n+12=42`
`(n-10)(n+n)=0`
`n=10`.


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