12. There is a well, at the bottom of which there is a spring that continuously gushes out water, with the same amount of water gushing out every minute. If 4 water pumps are used to pump out the water, it takes 40 minutes to finish; if 5 water pumps are used, it takes 30 minutes to finish. Now, it is required to pump out all the water within 24 minutes, at least $\qquad$ water pumps are needed.
【Analysis】Assuming 1 pump extracts 1 unit of water per minute, then the amount of water surging out per minute is $(4 \times 40-5 \times 30) \div(40-30)=1$ unit. The original amount of water in the well is $4 \times 40-1 \times 40=120$ units. To extract all the water in 24 minutes, the number of pumps required is $120 \div 24+1=6$ pumps.