They are testing you know what a tenth, or an eighth or a fith is.
I’m really not convinced they were.
Certainly not in the context of a set of questions about hundreds, tens, and units, (and tenths, hundreths etc).
That just seems to be what someone on Mumsnet decided.
You are confusing yourself with your ability to work out what 1 tenth of 1.5 is (and one tenth of 1 if you want to answer 15).
Nope.
The answer I expected was 5
e.g. simply the contents of the “tenths” column