Acertain bacterial culture is multiplying in a culture dish, doubling in size every 6 minutes. it takes exactly an hour to fill the whole dish. how long does it take to fill half the dish?

Doubling in size,

if the whole dish is filled in an hour (all of it)
how long does it take to fill 1/2 dish?

notice that
1/2 doubled=1/2 times 2=2/2=1 whole dish
therefor it only doubled one more time from the 1/2 point

1hour=all
1hour-6mins=1/2 dish

1hour-6mins=60-6=54 minutes

answer is 54 minutes