John and David can finish a job together in 3 hours. If John can do the job by himself in 4 hours, what percent of the job does David do?	Options.
A) 35%	B) 28%	C) 26%	D) 20%	E) 25%
You can also plug in numbers. For example, Bob and Alice work at a donut factory and make 12 donuts which is the job ( I picked this as a SMART number).   John on his own works  12/4= 
<gadget id="calculator"> 12/4</gadget>
<output>3</output>
3 donuts per hour.   John and David work   12/3= 
<gadget id="calculator">  12/3</gadget>
<output>4</output>
4 donuts per hour so David works 1 donuts/hour   To find out the percentage, David works 1 donuts/hr x 3 hours= 3 donuts per hour.   Therefore 3 donuts/12donuts = 1/4 = 25%   Answer : 25%
<result>E</result>
John and David can finish a job together in 3 hours. If John can do the job by himself in 4 hours, what percent of the job does David do?