Hi,

I think I am a bit late but hope to give you an alternative solving method.

Fist we tabulate the information as follows

We will then Substitute 40 screw with 80 nails.

60u = 900

1u = 15

20u = 15 x 20 = 300 (20 nails weigh 300g)

500 – 300 = 200g (Box)

This is a modification of the Simultaneous Equation substitution method.

This method can be used for container with liquid or sand.

20 nails weigh the same as 10 screws. 

The second set weighs lighter is logical since it effectively carries only “10 screws” compared to 40 screws in first set. 

The drop in 30 screws caused drop in weight by 900g (1.4kg – 500g). 

30 screws –> 900g

10 screws –> 300g

40 screws –> 1200g

mass of empty jar = 1400-1200=200g#

Since it is stated the screw weigh twice of nail, we can convert all
the nails to screw or vice versa.
Screw = 2x nail or nail = screw/2

Let's convert the nails to screw.
20nails weigh the same as 10 screws

10 screws + jar = 500g => 40 screws + 4 jar = 2000g ---(1)
40 screws + jar = 1400g   ----(2)

(1)-(2)
3 jar = 600g
1 jar = 200g ####