Esther read 4 books in 2 h 15 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 19 43. What time did she start reading the fourth book?
Duration that Esther took to read the first book = 1 u
Duration that Esther took to read the second book = 1 u
Duration that Esther took to read the third book = 1 u
Duration that Esther took to read the fourth book = 1 u + 51 min

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

4 u + 51 min = 2 h 15 min
4 u = 135 min - 51 min
4 u = 84 min  

1 u = 84 min ÷ 4 = 21 min

Duration that Esther took to read the fourth book
= 21 min + 51 min
= 72 min
= 1 h 12 min

1 h 12 min before 19 43: 18 31

Time that Esther started to read the fourth book = 18 31

Answer(s): 18 31