To calculate the net present value of studying, you need to consider:
- the cash flow over your working life if you study;
- the cash flow over your working life if you do not study.
You have the information to infer 1. But the information you provide does not enable you to infer 2, so you cannot calculate the net present value.
The key information that is missing is how much you would have been able to earn in each of the 48 years if you did not study. Suppose for example that you would have earned 25k per annum in each year. Then the effect of studying would be that in each of the first 3 years, you would be worse off by 25k + 10k - 5k = 30k, and in each of the next 45 years you would be better off by 40k - 25k = 15k. The net present value from studying, given a discount rate $x$, is:
$$NPV = \displaystyle\sum_{n=1}^3\bigg(\frac{-30}{(1+x)^n}\bigg) + \displaystyle\sum_{n=4}^{48}\bigg(\frac{15}{(1+x)^n}\bigg)$$
This assumes that all cash flows are at the end of the relevant year and would need adjusting if any are at the beginning of or during a year.