Question

in a practical unit if the unit of mass becomes double and that of time becomes half then 8 joule will be equal to.....unit of work

Answer 1

Joule = kg * m² / s²

Where kg is kilogram, m is meter and s is second.

Let new units be newJoule, newKg and newS. 
Then formula for newJoule would be newKg * m² / (newS)²

According to the question newKg = 2kg i.e. 1kg = 1/2 * newKg
newS = s/2 i.e. 1s = 2 * newS

So 8 Joule =  8 * kg * m² / s²
=  8 * ( 1 / 2 * newKg )  * m² / (2 * newS)² 
=  (8 * 1/2 * 1/4) * newKg * m² / (newS)²
= 1 * newKg * m² / (newS)²
= 1 newJoule

Answer 2

8joules of work done can be expressed,as
work= 8 joules = 8kg m²/ s²
where normal units mass = kg
length= m
Time = second=s
let the new units be KG,M and S
According to question ,new mass is double that of initial value.
KG = 2kg
M= no change mentioned = so assume same = m
S= s/2
work =8 kg m/ s
=8 (KG/2)*M²/(2S)²
=8/8(KGM²/S²)
=1 Joule of work.