Let's work with outcomes rather than probabilities. There are

possible outcomes for any given ticket.

There is 1 outcome where your chosen numbers match the jackpot. Now, let's count the outcomes where the ticket matches the winning numbers for at least five of the six numbers.

At least 5 winning numbers means exactly five winning numbers and one non-winning number or exactly six winning numbers. So, let's try that. You have six winning numbers. Choose five of them. You have 43 non-winning numbers. Choose one of them. That is the number of outcomes that give you exactly five winning numbers and one non-winning number. There are a total of

outcomes. Finally, there is exactly 1 outcome where you win the jackpot. That is a total of 259 outcomes for "at least five winning numbers".

So, the conditional probability is the number of outcomes where you win the jackpot divided by the number of outcomes where you have at least five winning numbers. That is

.