$3 \cdot 41$ To complete a certain task, A and B working together need 2 days; B and C working together need 4 days; C and A working together need 2.4 days. Then the number of days A needs to complete the task alone is
(A) 2.8 .
(B) 3 .
(C) 6 .
(D) 12 .
(China Junior High School Mathematics Correspondence Competition, 1988)
[Solution] Let's assume that A, B, and C can complete the work alone in $x$ days, $y$ days, and $z$ days, respectively. According to the problem, we have
$$
\left\{\begin{array}{l}
\frac{1}{x}+\frac{1}{y}=\frac{1}{2} \\
\frac{1}{y}+\frac{1}{z}=\frac{1}{4} \\
\frac{1}{z}+\frac{1}{x}=\frac{1}{2.4} .
\end{array}\right.
$$
$\left.\frac{1}{2}(1)+(2)+(3)\right)$ gives $\frac{1}{x}+\frac{1}{y}+\frac{1}{z}=\frac{7}{12}$,
(4) - (2) gives $\frac{1}{x}=\frac{4}{12}=\frac{1}{3}$.
$$
\therefore \quad x=3 \text {. }
$$

Therefore, the answer is $(B)$.