A task

 Such a task is in the tests on the GRE before admission to universities in the states. So here are some who have difficulties with this.. I solved it in 20 seconds logically, some people there can't cope. Here is the difference between Soviet education and school education in the United States., although it, certainly, depends on the individual.
Test yourself.

`` If one paints the wall behind 4 o'clock, and the other for 6 hours. How much will they paint the wall together?
Answers

1. 2h 24min
2. 3h 12min
3. 3h 44min
4.  4h 10min
5. 4h 33 min