Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: a , b and c can do a work in 90 , 45 and 6 days respectively . if they work together , in how many days will they complete the work ?
5
21.67 or approximately 22 days.