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)
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!
Breakeven point can be calculated by:
the equation is basically Fixed Costs = Profit * # of Units
In this case, price per unit minus cost per unit is actually already provided to us (profit of $64 per customer). So, all we need to do is divide fixed costs of $1B by $64, to get # of units of 15.62 million customers!