Question

A uniform harf metre rule is freely pivoted at the 15cm mark and it balances horizontally when a body of mass 40g is hung from the 2cm mark

Answer 1

You want the mass of the rule maybe?
I suppose you mean half a meter long and center of gravity at 25 cm
then moments about pivot point:
40 (15-2) = m (25-15)
40 * 13 = 10 m
m = 52 grams

Answer 2

Then draw a diagram to show the arrangement.