reto wrote:

Mike's pencil box contains 30 pens. 15 of the pens are red, and all of the others are either blue or black. If Mike were to choose a random pen from the pencil box, what is the probability that it would be blue?

(1) The probability that the pen will be red minus the probability that the pen will be black equals 0.3

(2) The probability that the pen will be red or blue is 0.8

Could anyone help me calculating the probabilities of picking a blue pencil? DS is clear for me, but the true calculation of the numbers...

When you pick just one thing from a group, probabilities are just proportions. So here, the probability of picking a red pen is 15/30 = 1/2, or in other words, 1/2 the pens are red, or 50% of the pens are red. So it's not really a probability question at all; it's just a ratio question.

We have a box where 50% of our pens are red, and the rest are blue and black. Statement 2 is slightly easier - it tells us that 80% are either red or blue, so if 50% are red, then 30% are blue. Similarly, Statement 1 tells us that 20% of the pens are black, so 30% will be blue.

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