P(win) = 1 / (2*3*4*5*6) = 1/720
We also need to calculate the chance of the game ENDING here - getting all wrong. That chance is:
P(loss) = (1/2)(2/3)(3/4)(4/5)(5/6) = 1/6
Now - here's another important piece of information that's missing; they tell you exactly what numbers are correct! The exact probability is really complicated to calculate, because it is state dependent. You need to look at all the possible cases (how many and which ones are right), and calculate the follow-on chances of winning or losing. It's a long, very detailed problem. For example, the chances of getting the last four right and the first wrong are also 1/720. If that happens, you'll win on the next round (changing the first digit to the only other possibility, the right one.) The chances of getting just the FIRST four right is 1/144, and you would have a 1/5 chance of winning at that point. You have to look at each situation independently, then add them up.
The last question is also quite complex. Let's find the probability of beating a 0.75 as the second player, assuming they'll STOP with a 0.75 on the first spin:
P(win) = 0.25 + 0.7 * 0.25 = 0.425
P(tie) = 0.05 + 0.7 * 0.05 = 0.085
P(lose) = 0.49
So the chances of winning outright (no ties) with a 0.75 would be:
(0.49)^2 = 24.01%
The chances of one player tying is:
2*0.49*0.085 = 0.0833, which would make the chance of winning 0.04165 (50% if tied).
The chances of both tying is:
(0.085)^2 = 0.007225, and the chance of winning would be 1/3 of this ~= 0.0024083...
So the total chance of winning is:
P(win) = 0.2401 + 0.04165 + 0.0024083.... = 0.284158333....
This is pretty complex - if this is for a school project, please study this long enough that you truly understand it. The second one is a real bear - you might need an Excel spreadsheet to calculate it.
Ed: Okay - since this is a school project I can't help you further than this ... I've already done too much. However I will say this - if it was MY school project, I would SCRAP question number two and come up with a different question. This is MUCH, MUCH too hard - I can do it, but it'd probably take ME a week to calculate the answer, with computer assistance. How about determining the best strategy for picking a dollar amount as one of the four contestants? Or determining expected winnings in the Plinko game?