Elizabeth has 7/10 of a dollar. if she spends 4/10 of a dollar,what fraction of a dollar will she have left?

The answer is (6/25). I just multiplied the 2 numbers, but I'll explain with something simpler:

Take the number 100, let's say I want (1/10) from that 100,

and from that (1/10), I want (4/5).

Well a tenth of 100 is 10, and four fifths of 10 is 8, thus we have 8/100 as a total (or 4/50)

The same answer can be found when multiplying (1/10) and (4/5), giving us (4/50) or (8/100)