Subtract 12L 435 mL from 18L 295 mL.

Hint: In mathematics, it is very important for the data given to be in the same unit so that we can carry out the desired and/or required algebraic as well as mathematical operations.
Here, in question two measuring units are involved, so we need to convert either of the quantities to another one and then subtract them to get the result.

Complete answer:
Here, in the question, two different units have been given, i.e., litres (L) and mili-litres (mL). So, the first task is to convert the given data in only one unit, i.e., either in litres only or in mili-litres only.

Let us convert litres to mili-litres by following the rule of conversion as: $1{\text{ }}L = 1000{\text{ m}}L$ in both the data.
$12L = 12000mL$ and, $18L = 18000mL$
Now, add the mili-litres quantity to get the net quantity as:
$12L{\text{ }}435mL = 12000mL + 435mL \\
= 12435{\text{ }}mL - - - - (i) \\$
Also,
$18L{\text{ 295mL = 18000}}mL + 295mL \\
= 18295mL - - - - (ii) \\$
According to question, subtract equation (i) from equation (ii) to get the result as:
$Difference = 18295mL - 12435mL \\
= 5860mL \\
= 5.86L \\$

Hence, the difference between the two quantities 12L 435 mL from 18L 295 mL is 5860 mL or 5.86 L.

Note: Students should be careful while converting one unit to another as if we move from smaller measuring unit to greater measuring unit then we have to divide the numeric value while if we move from greater to smaller measuring unit, then we have to multiply the numeric value.