a) $$28\frac{4}{7}{\text{ years}}$$
b) $$31\frac{5}{7}{\text{ years}}$$
c) $$32\frac{1}{7}{\text{ years}}$$
d) None of these
Answer: b
Explanation: Required average
$$\eqalign{
& {\text{ = }} {\frac{{67 \times 2 + 35 \times 2 + 6 \times 3}}{{2 + 2 + 3}}} \cr
& {\text{ = }} {\frac{{134 + 70 + 18}}{7}} \cr
& = \frac{{222}}{7} \cr
& = 31\frac{5}{7}{\text{ years}} \cr} $$
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