Alright, lets get started.
Given that George is working in garden on each 3rd day.
Maria is working in garden on each 4th day.
Then they will not work together on days that are common multiples of 3 and 4.
The least common multiple of 3 and 4 is 12.
Means they will work together after 12 days from today. : Answer
Hope it will help :)