Calculate the average mass number of copper on the basis of given information

The relative abundance of two isotopic forms of copper viz. Calculate the average mass number of copper on the basis of given information.

Average mass number = (Mass number ${}{63}^{29}Cu$ x 70 / 100) +
(Mass number ${}
{65}^{29}Cu$ x 30/100)
= 63x70/100 + 65 x 30 /100 = 63.6