exam Maths Puzzle Numerical Ability

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

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exam Other Question
For the home described in exercise 15, if the roof is 7 m from peak to
eave and the attic space is 3 m high at the peak, how long does each of the pieces
of insulation need to be? Round to the nearest tenth.

May you please show your work I got to find the constand of variation k



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