Answer:
2 days.
Step-by-step explanation:
Let, Sofia can type the x amount of work in 10 hours.
So, in one hour Sofia can type amount of work.
Again, Nova can type x amount of work in 15 hours.
So, in one hour Nova can type amount of work.
Hence, if they work together, they can type amount of work in one hour.
Therefore, working together they can type the x amount of work in days.
So, they have to work for (6 - 4) = 2 days more to finish the work. (Answer)