12. (5 points) $A, B, C, D$ four people are carrying a flashlight and need to cross a dark tunnel that only allows 2 people to walk at a time. Each time, 2 people take the flashlight to cross, then one person returns with the flashlight, and again 2 people take the flashlight to cross $\cdots$, if $A, B, C, D$ four people take $3, 4, 5, 6$ minutes respectively to cross the tunnel alone, then the minimum time required for all 4 to cross is $\qquad$ minutes.
【Analysis】To minimize the time for four people to pass, the person who takes less time to go through the tunnel should run back and forth, which leads to the conclusion.
【Solution】We discuss in two scenarios:
First scenario: $A$ and $B$ cross, $A$ returns, $4+3=7$ (minutes)
$C$ and $D$ cross, $B$ returns, $6+4=10$ (minutes)
$A$ and $B$ cross, 4 (minutes)
Total time used is $7+10+4=21$ (minutes);
Second scenario: $A$ and $B$ cross, $A$ returns, $4+3=7$ (minutes)
$A$ and $C$ cross, $A$ returns, $5+3=8$ (minutes)
$A$ and $D$ cross, 6 (minutes)
Total time used is $7+8+6=21$ minutes.
Therefore, at least 21 minutes are needed;
The answer is 21.