Let x be the jug, 
for half-filled, y for the weight of the jug when empty.
∵ x + y = 2.6 kg
∵ x = 4 kg
∴ ⋅ 4 + y = 2.6 kg
∴ 2 + y =2.6 kg
∵ y = 2.6 - 2 = 0.6 kg
∴ The weight of the empty jug = 0.6 kg
0.25 kilometers
Using the universal measurement
Kilo, hecto, deca, unit, deci, centi, milli
Kilometer, hectometer, decameter, meter, decimeter, centimeter, millimeter
From unit you would move three time to the left from your decimal point on your starting number 250. You would move it three times to the left because meter to kilometer is three decimals to the left
A way I remember my universal measurements is king (k-kilo), Henry (h-hecto), died (d-deca), unexpectedly (u-unit as in liters, meters, grams) drinking (d-deci), chocolate (c-centi) milk (m-milli)
