Harry and Ron can individually build a castle in 20 days and 30 days respectively. Both work together for 6 days, after which Harry and Ron got tired and quit. Hermione decided to complete the castle and worked for 5 days alone to complete the remaining work. Had Hermione worked alone, in how many days would she have built the whole castle?
a. 5 days
b. 10 days
c. 15 days
d. 20 days
e. 30 days
Answer: B
Source: e-GMAT