You don't add, you multiply. This isn't actually a Pokemon question, more a generic math question soo... idk if I should answer, but let's go
This is a measure of probability rather than percentages. First remember that probability can be expressed in 2 ways - in a percentage form (10% chance) or in a fraction form (1/10 chance). Percentages can also be expressed as a decimal
Wha do I mean you ask?
10% = 1/10 = 0.1
100% = 10/10= 1
Remember fraction rules - it doesn't actually need to be 10/10 or 1/10, it can be 2/20 which simplifies into 1/10.
To convert a fraction into a Percentage, multiply it by 100. So let's say, you want to convert 41/50 to a fraction form. Multiply it by 100 = 82. Therefore, it's 82%. Similarly, to convert a percentage to a fraction, divide by 100.
So let's say, you have a Pokemon that has a 50% chance of popping up.
You agree, 50% = 1/2 right?
So if you get 50% twice
= 1/2 x 1/2
You agree 1/4 = 25% yes?
So basically you have a 25% chance.
Here's a real life example if you don't really understand
You have a coin. You flip it twice.
The first time you could get Heads or Tails
The second time you could get Heads or Tails again.
How many possible combinations are there?
Well, you could get Heads, then Heads again, or Heads then Tails, or Tails then Heads or Tails then Tails. So there are 4 possibilities.
What is the chance of getting Heads both times?
You have 4 possibilities, and only 1 of them works. So that's 1/4 chance that you'll get 2 heads in a row. Multiply that by 100, = 25% chance.
For future reference, just ask on my wall or something if you want other help.