Nicole read 4 books in 1 h 30 min. She took the same amount of time to read each of the first 3 books. She took 50 min longer to read the fourth book than each of the first 3 books. She finished reading all the books at 16 12. What time did she start reading the fourth book?
Duration that Nicole took to read the first book = 1 u
Duration that Nicole took to read the second book = 1 u
Duration that Nicole took to read the third book = 1 u
Duration that Nicole took to read the fourth book = 1 u + 50 min

Total duration that Nicole took to read the four books 
= 1 u + 1 u + 1 u + 1 u + 50 min
= 4 u + 50 min

4 u + 50 min = 1 h 30 min
4 u = 90 min - 50 min
4 u = 40 min  

1 u = 40 min ÷ 4 = 10 min

Duration that Nicole took to read the fourth book
= 10 min + 50 min
= 60 min
= 1 h

1 h before 16 12: 15 12

Time that Nicole started to read the fourth book = 15 12

Answer(s): 15 12