Let r be the rate for ONE MAN, in the unit FIELDS per DAY. You would express and mean the rate as 1 field per number of days.
Let d be the unknown number of days for the question.
The description turned into equations:
The amount of hours per day ratio was included in each equation for the two separate parts of the description. The variable, d, is for TIME in DAYS. The question is also about time in DAYS.
To condense some of the notation about the units to have more easily-read system,
Solve for d.