Xiao Zhang is walking at a constant speed on the ring road. Every four minutes, a bus meets him head-on. Every 12 minutes, a bus of the same route runs from his back to his front. How many minutes does the bus stop send a bus?

Suppose every X minutes, the speed of the car a, the speed of the people B
1:ax/(a+b)=4
2:12a=12b+ax
The solution 1 is AX = 4A + 4B
2 is AX = 12a-12b
A = 2B
Substituting a = 2B into 1 leads to
2bx=12b
x=6

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