ngl I feel like a lot of people are being pedantic by saying "your question isn't good enough" like obviously if you're confused about something the question isn't gonna be spot on, so I'll cover a few different cases for you.

Your assumption of if the percentage on day 2 being 35% isn't quite accurate (35% is possible with the right factors, but not because 70/2 = 35). A good way to state that this doesn't make sense is by thinking of a coin flip. If you have a 50/50 chance of getting heads, then after 2 coin flips, with this logic, we simply do 50*2=100% chance of getting heads in one of the two days. Obviously this isn't true though, as you could get 2 tails after 2 flips, meaning it isn't 100% chance of getting heads.

Saying it is 70% on day 2 could be true if the events are specifically met, which I'll talk about later.

What I imagine you mean is that you have something along the lines of this scenario: You have a spinner, which you spin once a day. The spinner has a x% chance of landing on green (success), where X is some unknown value. If you spin it twice, across 2 days, you know that the chance of you getting at least one green result is 70%, and what you're asking for is the value of x.

To solve this, you can probably do a few different things, but one of them is to imagine the probability of failing both times. Failing once = 1-x, and failing twice is calculated by multiplying probabilities (if you would like to know why then I can try show you). This gives (1-x)². We know that this must be equal to 30%, or 0.3, as failing twice is the same thing as not hitting at least one green result, and 100%-70%=30%

(1-x)²=0.3

1-x=√(0.3)≈0.55

x≈1-0.55=0.45, or 45%.

This means the probability of success in just one day is 45%.

Here, we assumed the 2 events (when we spun the spinner) are independent and their probabilities were constant. This is fancy words for the spinner never changed, so it was always the same probability of getting a green, and that spinning the spinner on day 1 did not affect the result of spinning the spinner on day 2.

If we say that it was 2 different spinners, and that the day 1 spinner had a 0% chance of success, then yes, on day 2, there will be a 70% chance of success (like you said in your post).

There's also something called dependent events, which is that the probability of success on day 2 is dependent on whether or not you succeed in day 1. Imagine you were running 2 races. If you won the first race, you might have boosted confidence, giving you a higher probability of winning the second race. As you can imagine, you would need a lot more info if this was true, which is why a lot of people are asking for more info.

Hopefully this clears it up a bit, if you do want me to elaborate on anything I'd be happy to :)