Irene read 4 books in 2 h 38 min. She took the same amount of time to read each of the first 3 books. She took 38 min longer to read the fourth book than each of the first 3 books. She finished reading all the books at 20 42. What time did she start reading the fourth book?
Duration that Irene took to read the first book = 1 u
Duration that Irene took to read the second book = 1 u
Duration that Irene took to read the third book = 1 u
Duration that Irene took to read the fourth book = 1 u + 38 min

Total duration that Irene took to read the four books 
= 1 u + 1 u + 1 u + 1 u + 38 min
= 4 u + 38 min

4 u + 38 min = 2 h 38 min
4 u = 158 min - 38 min
4 u = 120 min  

1 u = 120 min ÷ 4 = 30 min

Duration that Irene took to read the fourth book
= 30 min + 38 min
= 68 min
= 1 h 8 min

1 h 8 min before 20 42: 19 34

Time that Irene started to read the fourth book = 19 34

Answer(s): 19 34