ZSE Co – Joint probabilities

Hello Sir
There is a question titled ZSE Co in BPP & Kaplan exam kits on joint probabilities. There is a capital of $500,000 withdrawn at the start which they have deducted from all the cash flows instead of just deducting it from the whole total in the first line. I cannot comprehend the answer given in the kit.

Could you please explain the concept of joint probabilities.
Thanking you a million.