SOLUTION: Robot A takes 6 minutes to weld a frame. With the help of Robot B, the job can be done in 4 minutes. How long would it take Robot B working alone?

Make this chart

                Number of frames welded | time | rate in frames/minute
Robot A alone                           |      | 
Robot B alone                           |      |
----------------------------------------------------------------------
Both together                           |      |

The question asks:

Let the answer to that be x.  It would take Robot B x minutes alone to
weld 1 frame.  So fill in 1 for the numer of frames Robot A alone could 
weld and x for the time:


                Number of frames welded | time | rate in frames/minute
Robot A alone                           |      | 
Robot B alone             1             |   x  |
----------------------------------------------------------------------
Both together                           |      |

Since robot A takes 6 minute to weld 1 frame, fill in
1 for the number of frames and 6 for the time for Robot A
alone:

                Number of frames welded | time | rate in frames/minute
Robot A alone             1             |   6  | 
Robot B alone             1             |   x  |
----------------------------------------------------------------------
Both together                           |      |

Since it takes 4 minute for both together to weld 1 frame, fill in
1 for the number of frames and 4 for the time for both together:

                Number of frames welded | time | rate in frames/minute
Robot A alone             1             |   6  | 
Robot B alone             1             |   x  |
----------------------------------------------------------------------
Both together             1             |   4  |


Now finish filling in the chart by calculating the three rates in
frames per minute by dividing the number of frames by the number
of minutes:

                Number of frames welded | time | rate in frames/minute
Robot A alone             1             |   6  |         1/6 
Robot B alone             1             |   x  |         1/x
----------------------------------------------------------------------
Both together             1             |   4  |         1/4

Now we make the equation by this:

     Robot A's rate alone + Robot B's rate alone = their rate together:

                1/6       +           1/x        =        1/4



Multiply thru by 12x





It would take Robot B 12 minutes to weld one frame.

Edwin