There is a bag filled with 5 blue and 6 red marbles. A marble is taken at random from the bag, the colour is noted and then it is replaced.
Another marble is taken at random.
What is the probability of getting at least 1 red?

There are 6+5 marbles.

Of those, 5 are red.

The chances of drawing a red marble are 5/11

Since you replace the marble you drew, the chances each draw remain 5/11

You cannot draw two red marbles without drawing a red marble on your first draw, so the odds of you making a second draw are 5/11. The odds of that also being red are 5/11… but we only care about that 5/11ths the time

5/11*5/11=25/121