acoording to your question
let the relative abundundance in decimal be x y and z for M,M+1,M+2 respectively
then x+y+z=1
given
Mx +(M+1)y +(M+2)z=M+.5
y+2z=.5
no other thing is given and there can be various ratio possible base on only 1eqn.as the average relative mass is close to M therefore it should have large mole of ratio.
thanks and regards
sunil kr
askIItian faculty