PS work and rate tractors

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Posted by: dauntless18

Four tractors working together can plow a field in 12 hrs. How long it will take 6 tractors to plow a field of the same size, if all tractors work at the same rate?

A. 6hr
B. 9hr
C. 10hr
D. 18hr.
E. 8hr

IMO E

explnantion;
4 tratctors plows in 12 hr
so in 1Hr four tractor can plow
1/12..field
so 6 tractor can plow
4 ~ 1/12
6 ~ X
so X = (6 * 1/12)/4
X = 6/48
X = 1/8
so in 1 hr 1/8 field to Plow complete field it will take 8 hrs