DAY 71

Comment

This can be solved by trial and improvement (though this is not entirely straightforward, in part because the solution is not a whole number).
It can also be tackled more formally, by solving the equation n + 9 = 3n.

Answer

n = 4.5.

Readers' methods

L Carr, Didcot:
I tried different values for n:
3 gives a 12 by 9 rectangle - too tall;
2 gives an 11 by 6 rectangle - even worse;
4 gives a 13 by 12 rectangle - better but still too tall;
5 gives a 14 by 15 rectangle - now too wide;
41/2 gives a 131/2 by 131/2 rectangle - a square!

xxx