There are 18 points in a plane of which 5 are collinear. How many straight lines can be formed by joining them?

How many $3$ digit numbers can we make using the digits $4,5,7$ and $9$ and where repetition is allowed?

Number of six digit numbers whose sum of the digits is $49$ are 

${ if } ^ { n }{ C }_{ 3 }{ + }^{ n }{ C }_{ 4 }>{ n+1 }_{ C_{ 3 } }$ then