EgmatQuantExpert wrote:

Solve any Mean and Standard deviation question under a minute- Exercise Question #11- In a class of 34 students, the averages age of all the students and class teacher combined is ‘b’ years. Now, if the class teacher, age 43, retires in the same year and is replaced by experienced teacher age 68. Then what will be the new average of the class including the class teacher?

Options a) b+\(\frac{5}{9}\)

b) 7b+5

c) b-\(\frac{2}{3}\)

d) b+\(\frac{5}{7}\)

e) b+\(\frac{7}{5}\)

LOGIC will get you to the answer immediately..numbers remain same = \(34+1=35\) ...

only \(68-43=25\) years are added to total

so \(\frac{25}{35}\) will be added to the earlier average b...

ans \(b+ \frac{25}{35} = b+\frac{5}{7}\)

substitution..take b as 10, so total age is 35*10=350

add 68 and subtract 43 because of changeover so 350+68-43=375

new average =\(\frac{375}{35}= 10\frac{25}{35}= 10+\frac{25}{35} = 10+\frac{5}{7}\)

now substitute b as 10 and D is the answer

Algebraic methodtotal age = 35*b

add 68 and subtract 43 because of changeover so \(35b+68-43=35b+25\)

new average = \(\frac{35b+25}{35}\frac{[}{fraction] = b+[fraction]}5/7\)

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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372

2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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