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.