Gabby read 4 books in 3 h 11 min. She took the same amount of time to read each of the first 3 books. She took 51 min longer to read the fourth book than each of the first 3 books. She finished reading all the books at 16 33. What time did she start reading the fourth book?
Duration that Gabby took to read the first book = 1 u
Duration that Gabby took to read the second book = 1 u
Duration that Gabby took to read the third book = 1 u
Duration that Gabby took to read the fourth book = 1 u + 51 min

Total duration that Gabby took to read the four books 
= 1 u + 1 u + 1 u + 1 u + 51 min
= 4 u + 51 min

4 u + 51 min = 3 h 11 min
4 u = 191 min - 51 min
4 u = 140 min  

1 u = 140 min ÷ 4 = 35 min

Duration that Gabby took to read the fourth book
= 35 min + 51 min
= 86 min
= 1 h 26 min

1 h 26 min before 16 33: 15 07

Time that Gabby started to read the fourth book = 15 07

Answer(s): 15 07