Opal read 4 books in 2 h 34 min. She took the same amount of time to read each of the first 3 books. She took 30 min longer to read the fourth book than each of the first 3 books. She finished reading all the books at 20 20. What time did she start reading the fourth book?
Duration that Opal took to read the first book = 1 u
Duration that Opal took to read the second book = 1 u
Duration that Opal took to read the third book = 1 u
Duration that Opal took to read the fourth book = 1 u + 30 min

Total duration that Opal took to read the four books 
= 1 u + 1 u + 1 u + 1 u + 30 min
= 4 u + 30 min

4 u + 30 min = 2 h 34 min
4 u = 154 min - 30 min
4 u = 124 min  

1 u = 124 min ÷ 4 = 31 min

Duration that Opal took to read the fourth book
= 31 min + 30 min
= 61 min
= 1 h 1 min

1 h 1 min before 20 20: 19 19

Time that Opal started to read the fourth book = 19 19

Answer(s): 19 19