Practice30Create account

A spinner has four equal sectors, exactly one of which is red. It has just been spun four times and has stopped on red every time. What is the probability that the fifth spin also stops on red?

Competition practiceData & Probability
  • A1/4, because the spinner has no memory of the spins that came before
  • B1/1024, because that is the chance of five reds in a row
  • CLess than 1/4, because red is now due to stop appearing
  • DMore than 1/4, because the spinner has shown that it favours red

Step-by-Step Solution

The spins are independent, so what has already happened cannot alter what the spinner does next: the fifth spin stops on red with probability 1/4, exactly as the first did. The value is the probability of five reds in a row BEFORE any spinning starts; once four reds have already been seen, four of those five factors have been settled and only the last one is still uncertain. 'Red is due to stop' is the gambler's fallacy — a fair spinner keeps no account to settle, and the long-run balance is restored by later spins swamping the early ones, not by them being corrected. 'The spinner favours red' would be a fair suspicion about a spinner that might be biased, but the question states that the four sectors are equal, so the model is given and four spins is far too little evidence to overturn it.
Answer: A) 1/4, because the spinner has no memory of the spins that came before

Master this topic in 30 minutes a day.

Don't just memorise the answer — build a daily practice habit.

Practice 5 Similar Questions Free →

All Data & Probability practice questions

More Data & Probability practice

Eight hundred people were asked how often they use a library. The relative frequencies…Bag A holds 3 red and 2 white balls; Bag B holds 2 red and 4 white balls. One ball is…The marks of 25 students are recorded as — mark 10: 3 students, mark 20: 5 students,…A fair die is to be rolled several times. What is the SMALLEST number of rolls for which…For the five readings 3, 8, 11, 15 and 23, the quantity (3 − a)² + (8 − a)² + (11 − a)²…