Answer:
4.5 minutes
Step-by-step explanation:
1 problem in 3 minutes is equivalent to a rate of 1/3 problem per minute, when their work together.
Let's call m to Marco's rate of work. Then Cliff's rate of work is m/2. The addition of Marco's and Cliff's rate of work make their combined rate of work, that is:
m + m/2 = 1/3
3/2*m = 1/3
m = 1/3 * 2/3 = 2/9 (2 problems in 9 minutes)
Then, it takes to marco 9/2 = 4.5 minutes to solve a problem
