Finally to talk about 0.2 of the square means that the square itself is a measure unit. When the whole is partitioned into tenths only, students need to only unitise once. There is only one measure unit invoked. Similarly if hundredths only are to be considered. This depth of cognitive processing underlies the difficulties that students have with the basic place value ideas and can be a cause of students' resorting to the simpler but erroneous misconceptions .
Many children have difficulty deciding how or whether zeros change the value of a number.
The essential understanding of which zeros affect the place value of the digits and which ones do not is best demonstrated using concrete materials such as Multi Attribute Blocks. Making 3.25 and then 3.025, 3.250, 3.205 and 03.25 from blocks clearly demonstrates which zeros affect the size of 3.25, rather than just providing a list of rules for students to learn. Using the ideas of expanded notation are crucial. It is very hard for children who do not understand the place value basis of decimal numbers to memorise the rules. Children generally decide which zeros change the value of a number according to their own interpretations of decimal notation.