I have £1.88 in my pocket. All the coins are different. I take one out. What is the probability that I have taken out a 20p coin?

To get the answer for this, draw first a tree diagram.

1 x £1

1 x £0.50

1 x £0.20

1 x £0.10

1 x £0.05

1 x £0.02

1 x £0.01

= £1.88

so probability of taking out a 20p coin is 1/7 for the first attempt.

But if you have more than one attempt, still make a tree diagram so you can easily see the probabilities.