5/10 as a unit fraction is 1/2, but 4/5 cannot be expressed as a single unit fraction, but can be expressed as 1-1/5, where 1/5 is the unit fraction. 1/2 can be expressed as 1-1/2. 4/5 can also be expressed as the sum 1/5+1/5+1/5+1/5 and 5/10 can be similarly expressed as 1/10+1/10+1/10+1/10+1/10. 1/5 can be expressed as 1/10+1/10, so 4 pairs of these adds up to 4/5.