Xiao Zhang's math teacher got full marks in math.

Because the units are different, let's convert them into hours first:

Xiao Wang: One third of an hour (1/3 hours)

Xiao Zhang: Three tenths of an hour (3/ 10 hour)

Xiaoxin: quarter of an hour (1/4 hours)

Denominators are different, so it's hard to compare, right?

Ok, let's divide it again:

Xiao Wang: 1/3 hours =20/60 hours (20th/60th hour).

Xiao Zhang: 3/ 10 hour = 18/60 hour (18th 60th hour).

Xiaoxin: 1/4 hours = 15/60 hours (the first 15/60 hours).

So we are more alike!

So we can get 20/60 hours (Xiao Wang) > 18/60 hours (Xiao Zhang) > 15/60 hours (Xiao Xin).

That is 1/3 hours (Xiao Wang) > 3/ 10 hours (Xiao Zhang) > 1/4 hours (Xiao Xin).

Because Xiao Wang is the oldest, it means that he spends the most time; Xiaoxin is the youngest, so she spends the least time.

So-Xiaoxin's homework is efficient.

Xiaoxin not only got full marks in the exam, but also spent the least time, so he was efficient and smart! Are you clear? :-D)