Recent case interview, I was asked to calculate how many battery our client's battery can save for their customers.

Background: client manufactures battery for forklifts. Their battery has outstanding quality and duration lifetime. The battery can be continuously used for 12 hours and needs to be charged for 8 hours. Their competitors' battery can be used for 10 hours and needs to be charged for 10 hours. Question: how many batteries the customers can save by using our client's battery.

There are definitely other factors I've examined e.g. extra energy saving by using our client's battery. However, I was stuck in the calculation of the above question. Could anyone help me:
(1) figure out the CORRECT calculation?
(2) suggest some cases either from preplounge or casebooks where I can practise on this type of math?

MBB interviews ahead. needs to improve it.

Thank you!