exam Maths Puzzle Numerical Ability

I got to find the constand of variation k

m varies directly with n; m=144 when n = 8

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exam Other Question

I have to simplify this; sqrt20/5-1/sqrt5

I keep coming up with that this cannot be simplified, am I on the right track.
my result is
=15sqrt5/100
please help Assume 15 people are in a pool of contestants. A panel of 3 contestants will be selected. What is the probability of a member of the pool of 15 being selected on the panel of 3?