The first step is to collect as many observations as possible about the problem you are trying to examine... if it could be proven wrong,then you have formed a hypothesis!
The atomic mass would be 28.08535 amu. Multiply 27.9769 by .92297 = 25.803. Multiply 28.9765 by .046832 to get 1.357. Multiply 29.9738 by .03872 to get .925351136. Add 25.803 + 1.357 + .03872 to get 28.08535 amu