Let they met at X ... Which is somewhere between A and B....
When they met.....one ran from A to X... another ran from B to X... together they ran da whole lap in 12 minutes.....
Now after they met.....
......one will run from X to B... another will run from X to A...then both will turn around and run again and meet at Y for da second time..... Y is somewhere between A and B....
So....AFTER they first met at X and UNTIL they met second time at Y....
.... together they covered = [ X to B ] + [ B to Y ] + [ X to A ] + [ A to Y ] = [ A to B ] + [ A to B ] = 2 [ A to B ] = 2 whole lap
................ because ................[ X to B ] + [ X to A ] = [ A to B ]... and.... [ A to Y ] + [ B to Y ] = [ A to B ] .....
So... After they met at X... they ran 2 laps and met at Y for da second time ....
And... They need 12 minutes to run a lap together....so they'll need 24 minutes to run 2 laps together....
so.... they met at Y ... 24 mins after they met at X ...and they met at X ... 12 mins after da start...
So... They met at Y ...in other words....they met again..... 24 + 12 = 36 mins after da start....