CFAI EOC Reading 61 #17

I understand the qualitative explanation of the answer, i.e. since delta < 1, number of calls needed to hedge will be higher than number of underlying asset. Hence A is the only possible answer. But I dont understand how to quantitatively get to the 815 shares and 1000 calls? In the explanation, it says “Assume one borrows the present value of $42.50 at time zero”. What is this the present value of?

From the binomial model, the 42.50 is 15% down from the $50 stock price.

So present value at time of expiration, one year later? What’s the logic behind starting with that to find the number of calls/shares?

Any more thoughts on how to tackle such questions?