Subtract 2.6 from 4 to get the weight of 1/2 the marbles:
4-2.6=1.4
Double (or multiply by 2) to get the weight of all the marbles (without the jar):
1.4+1.4=2.8
Subtract 2.8 from 4 to get the weight of the jar:
4-2.8=1.2
The empty jug would weigh 1.2kg.
