Acupboard contains 20 boxes of chalks which are red and blue in colour. a box of chalks is chosen at random from the cupboard. if the probability of choosing a box of blue chalks is 1/4 ,calculate 1)the number of boxes of red chalks in the cupboard2) the number of boxes of blue chalks that should be put into the cupboard so that the probability of choosing a box of blue chalks is 2/5

Let say that there are boxes with red chulk, and boxes with blue chulk. The total is obviously boxes.

If the probability of choosing a box of blue chalks is , it means that

which implies

So, there are blue boxes and red boxes (remember that the total is ).

Now, suppose that we add new boxes with blue chulk. The number of blue boxes becomes , and the total number becomes . The new probability of having a blue box is if

Cross multiply the denominators:

Expand:

Bring all -like terms to the left and all constants to the right:

Simplify:

So, you should add more boxes of blue chulk.

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