Question 83272
If sam can mow the lawn in twenty minutes, he can mow 100% in 20 minutes.  So, 1 minute is 5%.  Bob can mow 100% in 30 min but... 30 can't go into 100 evenly so we have to make it a percentage of something divisble by 30 and 20 in the hundreds so you can do the 300s.  Since 30 min is 300%, 1 minute is 10%.  Since we changed the 1st one too... it is 300% in 20 minutes and so 1 minute is equal to 15% because 300/20=15.  Together, in 1 minute, they should finish 10% + 15% which is 25% and 25% can go into 300% evenly.  It can go into it 12 times.  So the answer is 12 minutes.