Mary read 3 books in 2 h 26 min. She took the same amount of time to read each of the first 2 books. She took 38 min longer to read the third book than each of the first 2 books. She finished reading all the books at 21 20. What time did she start reading the third book?
Duration that Mary took to read the first book = 1 u
Duration that Mary took to read the second book = 1 u
Duration that Mary took to read the third book = 1 u + 38 min

Total duration that Mary took to read the three books 
= 1 u + 1 u + 1 u + 38 min
= 3 u + 38 min

3 u + 38 min = 2 h 26 min
3 u = 146 min - 38 min
3 u = 108 min  

1 u = 108 min ÷ 3 = 36 min

Duration that Mary took to read the third book
= 36 min + 38 min
= 74 min
= 1 h 14 min

1 h 14 min before 21 20: 20 06

Time that Mary started to read the third book = 20 06

Answer(s): 20 06