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There are 20 balls in a bag. The probability is 0.25 of picking a red ball. If one more red ball is added, what is the new probability of picking a red ball at random?

Convert 0.25 into a fraction where 20 is the denominator as there are 20 balls in total in the bag. 0.25 is the same as 1/4 and to convert this to 20 as the denominator multiply both the top and the bottom by 5 so the new fraction is 5/20. As the new ball to be added is red it is within both the numerator and denominator so add one to both to get the new probability of 6/21. Simplify this if possible (although this example 6/21 is the simplest form). As the question has given the original probability in a decimal, convert the fraction to a decimal to get 0.2857.

Alex L. 13 Plus  Maths tutor, GCSE Maths tutor, 11 Plus Maths tutor

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