A can do a piece of work in 10 days. He works at it for 4 days and then B finishes it in 9 days. In how many days can A and B together finish the work?
Choices:  A) 6  B) 7  C) 9  D) 5  E) 3
4/10 + 9/x = 1 =&gt; x = 15   1/10 + 1/15 = 1/6 =&gt; 6 days   6nswer: 6
<result>A</result>
A can do a piece of work in 10 days. He works at it for 4 days and then B finishes it in 9 days. In how many days can A and B together finish the work?