General Mathematics Paper 2,Nov/Dec. 2010
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Question 5

1. The number of green (G), red (R), white (W) and black (B) identical balls contained in a bag is as shown in the table.
 Balls G R W B Frequency 2 4 3 1

If two balls are selected at random without replacement, find the probability that both balls are green.

1. In a test, if a student had scored 80 marks in one of the subjects, his average mark in 8 subjects would be 62. If he had scored 64 marks in that same subject with the scores in the remaining 7 subjects unchanged, the average mark would be m. Find the value of m.

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Observation

This question was reported to be quite popular among the candidates and they performed well in it.
In part (a), while majority of them were reportedly able to obtain the individual probabilities correctly, most of them added them together instead of multiplying them. Others were not able to differentiate between selection with replacement and without replacement. Candidates were expected to show that probability that both are green = P(first is green and second is green) =  =  or 0.022.
In part (b), majority of the candidates were reported to have performed well in this question however, some candidates were reported not to be able to evaluate the simple arithmetic involved correctly. Candidates were expected to show that the total in the seven subjects that were constant was equal to 496 – 80 = 416. When the score of 64 is now added, the total would be 480. This, therefore, implied that the required average was
= 60.