The pressure difference between two places with different height can be calculated by using Bernoulli's principle. The principle can be used to find moving fluid, but since the water inside well is static we can remove the part with velocity. You also need to know the density of water, which is 1000 kg/m3. Assuming gravity unit is 9.8 m/, the calculation will be:
In order to maintain the same price after two days, the stock must go up (U) on two days and go down (D) on two days, the sample space for this event is:
S={UUDD, UDUD, UDDU, DDUU, DUDU, DUUD}
There are 6 equally likely possible outcomes. The probability that the price of the stock will be the same as it is today is: