Question 311608
Let rounding up the carts be 1 job.

Reginald takes 12 minutes for 1 job.
So he does 1/12 of the job in 1 minute.
..
Norman takes 18 minutes for the same job.
he does 1/18 of the job in 1 minute.
..
Together they do 1/18 +1/12 of the job in 1 minute.
= 2 +3 /36 
=5/36 of the job in 1 minute.

so to complete the job they take 36/ 5 minutes.
=7.2 minutes