@gizmotech I would follow this approach:
First ask relevant questions:
1. What type of bat is that?
2. What geographic location are we talking about?
3. To what age group are we catering to? Because the bats can be a plastic bat for small kids
4. Are we also serving professional sportsmen?
Once we get the answers for those questions, we can try to solve it:
1. Currently what percentage of people use it:
a. Urban - (5 to 10 age group) - x1%, (10 to 15 years) - y1%, (15 and above) - z1%
b. Rural - (5 to 10 age group) - x2%, (10 to 15 years) - y2%, (15 and above) - z2%
We can get the approximate percentages by surveying some schools, training institutes and professional clubs
2. What is the growth prospect:
a. Change in Population - p%
b. Increased awareness of a particular sport through branding - i%
c. Decreased popularity due a sporting mishap - d%
Additional info:
Total Urban Population: Pu
5 to 10 age group: Pu1
10 to 15 age group: Pu2
15 and above group: Pu3
Total Rural Population: Pr
5 to 10 age group: Pr1
10 to 15 age group: Pr2
15 and above group: Pr3
Ideally the approximation is:
Equation 1 (for Urban Population) = x1%*Pu1+y1%*Pu2+z1%*Pu3
Equation 2 (for Rural Population) = x2%*Pr1+y2%*Pr2+z2%*Pr3
Equation 3 (Percentage change) = (Equation1+Equation2) *p%
Total number of bats that can be sold is = Equation 1 + Equation 2 + Equation 3
We can use the other factors such as increased awareness or decreased popularity to even fine tune the prediction.
We can take the costs of each bat c1, c2, c3 to calculate the monetary size of the market
Ideally, this should be the approach of solving a case. Never ever predict any number from the back of your mind. Always say this is the method of doing this unless you have been given with a number by the interviewer.