Numbered discs are placed in a box and one disk is selected at random. if there are 7 red discs numbered 1 through 7, and 4 yellow disks numbered 8 through 11, find the probability of selecting a disk numbered 3, given that a red disk is selected

The chance of drawing a 3 given that the disc is red is 1/7.

In this example, we are told that one of the conditions is that the disc is red. There are only 7 red disks. So our chance is out of 7.

Of those 7 discs, only 1 has the number 3. Our chance is 1/7.