A four-digits number abba is divisible by 4 and a < b. How many such numbers are there?

If $ \left({x^2} + {{ 1} \over x^2}\right) = 6$, and 0 < x < 1, what is the value of $ {x^4} - {{ 1} \over x^4}$?

A boat goes 20 km upstream and 44 km downstream in 8 hours. In 5 hours, it goes 15 km upstream and 22 km downstream. Determine the speed of the boat in still water.

4 men’s work is equal to 6 women’s work, and 4 women’s work is equal to 6 boys’ work. A boy can finish the work in 60 days. In how many days can the work be finished by a man and a woman together?