One robot builds a robot in 1 hour, another one builds a robot in 2 hours. The 2 robots work together and when a new robot is complete, it joins the others, working at a constant rate of one robot in 2 hours. How much time will it take until there are 4 robots altogether, if the robots build only one robot at a time?
Choose the most appropriate option
A) 70 min	B) 94 min	C) 110 min	D) 112 min	E) 120 min
First the 2 robots work at the rate of   1 + 1/2 = 3/2   so they complete one robot in 2/3 rd of an hour = 40 minutes - (1)   Now the 3 robots work together at the rate of   1 + 1/2 + 1/2 = 4/2= 
<gadget id="calculator">4/2</gadget>
<output>2</output>
2/1   so they complete one robot in 1/2 an hour,i.e 30 minutes - (2)   70 minnd now we have 4 robots   So Total= (1) + (2) = 70 minutes   70 minnswer - 70 min
<result>A</result>
One robot builds a robot in 1 hour, another one builds a robot in 2 hours. The 2 robots work together and when a new robot is complete, it joins the others, working at a constant rate of one robot in 2 hours. How much time will it take until there are 4 robots altogether, if the robots build only one robot at a time?