Question

what is average mass number? explain with help of example. PLEASE

Answer 1

The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope)
 
Example

 A sample of element X contains 100 atoms with a mass of 12.00 and 10 atoms with a mass of 14.00. Calculate the average atomic mass (in amu) of element X.

Solution:

1) Calculate the percent abundance for each isotope:

X-12: 100/110 = 0.909 
X-14: 10/110 = 0.091

2) Calculate the average atomic weight:

x = (12.00) (0.909) + (14.00) (0.091)x = 12.18 amu

Answer 2

I'm not sure there is an average mass number
You can calculate the average atomic mass sure, but the mass number is a whole number quantity which tells us the number of nucleons in an atom of an element
Different isotopes will have different mass numbers but you can't have an average mass number as it has to be a whole number