
Question 6

 In a physics examination, the mean mark of the first twelve students in a class is 60, that of the next twenty students is 50 and that of the remaining y students is x. What is the mean mark for the whole class in the examination, in terms of x and y.
 A box contains 4 red and 3 blue identical balls. If two balls are picked at random, one after the other without replacement, find the probability that one is red and the other blue.

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