For this problem in particular, but also in general when it comes to production cost, think of break even always from the equation:
Fixed cost / (Price per unit - Cost per unit)
- Fixed costs: overall fixed costs for running the production (e.g., administration, plan, electricity, etc.) They need to be allocated to each product produced, in a fraction
- Price per unit: price at which each unit is sold
- Cost per unit: on top of the fixed ones, there are other costs, variable costs, associated to the price of producing each unit (e.g., raw material)
Hence, here we would need to divide $1B of fixed costs by $64, to obatin number of units of 15.6 million customers.
Hope it helps!