Let efficiency of a bullock and a tractor be B unit/day and T unit/day respectively.
Work done by X bullocks and Y tractors in 8 days to plough a field :
Work done by X bullocks = X * B unit/day * 8 days
Work done by Y tractors = Y * T unit/day * 8 days
Total work done = (X * B * 8 + Y * T * 8) units
Work done by X / 2 bullocks and 2 * Y tractors in 5 days to plough a field :
Work done by X bullocks = (X / 2) * B unit/day * 5 days
Work done by Y tractors = 2 * Y * T unit/day * 5 days
Total work done = (X * B * 2.5 + Y * T * 10) units
Both bullocks and tractors are used to plouge same field.
Therefore,
(X * B * 8 + Y * T * 8) units = (X * B * 2.5 + Y * T * 10) units
On solving, Y * T = 2.75 * X * B
Let X bullocks take m days to plough a field
Total work done = Work done by X bullocks
= X * B unit/day * m days
= (X * B * m) units
Here X bullocks plough the same field
Therefore,
(X * B * m) units = (X * B * 8 + Y * T * 8) units
X * B * m = X * B * 8 + (2.75 * X * B ) * 8
On solving, m = 30
Note: In question, Number of bullocks are halved in second statement.