28 machines can do a work in 10days. How many machines are needed to complete the work in 40days?  Choose the correct choice from the following answers.
A) 10 B) 6 C) 4 D) 7 E) 5
Required number of machines = 28*10/40= 
<gadget id="calculator">28*10/40</gadget>
<output>7</output>
7   Answer is 7
<result>D</result>
28 machines can do a work in 10days. How many machines are needed to complete the work in 40days?