A farming field can be ploughed by 6 tractors in 4 days. When 6 tractors work together, each of them ploughs 120 hectares a day. If two of the tractors were moved to another field, then the remaining 4 tractors could plough the same field in 5 days. How many hectares a day would one tractor plough then?  Choices.
A) 111 B) 184 C) 122 D) 144 E) 166
If each of 6 tractors ploughed 120 hectares a day and they finished the work in 4 days, then the whole field is: 120*6*4=720*4= 
<gadget id="calculator">720*4</gadget>
<output>2_880</output>
2880hectares. Let's suppose that each of the four tractors ploughed x hectares a day. Therefore in 5 days they ploughed   5*4*x=20*x hectares, which equals the area of the whole field, 2880 hectares.   So, we get 20x=2880   x=2880/20= 
<gadget id="calculator">2880/20</gadget>
<output>144</output>
144. Hence, each of the four tractors would plough 144 hectares a day.   Answer is 144.
<result>D</result>
A farming field can be ploughed by 6 tractors in 4 days. When 6 tractors work together, each of them ploughs 120 hectares a day. If two of the tractors were moved to another field, then the remaining 4 tractors could plough the same field in 5 days. How many hectares a day would one tractor plough then?