In a group of seven friends, the mean (average) age of three of the friends is 12 years and 3 months and the mean age of the remaining four friends is 13 years and 5 months. In months, the mean age of all seven friends is
(A) 156
(B) 154
(C) $155 \frac{1}{2}$
(D) 157
(E) 155
The mean age of three of the friends is 12 years and 3 months which is equal to $12 \times 12+3=144+3=147$ months.

Since the mean equals the sum of the ages divided by 3 , then the sum of the ages of these three friends is $3 \times 147=441$ months.

The mean age of the remaining four friends is 13 years and 5 months or $12 \times 13+5=156+5=161$ months.

Thus, the sum of the ages of these four friends is $4 \times 161=644$ months.

The sum of the ages of all seven friends is $441+644=1085$ months, and so the mean age of all seven friends is $\frac{1085}{7}=155$ months.

ANswer: (E)