It takes five minutes to pass a rumour from one person to two other persons. The tree of rumour continues. Find how many minutes does it take spread the rumour to 729 persons ?

After 5 mins ..... 3 persons will get rumour
After 10 mins .... 3x2 + 3 = 9 = $32$ persons will get
after 15 =5x3 min ......9x2 + 9 =27 = $33$ persons will get news.
.
.
after 5x6 mins ... $36$ = 729 persons will get news.

Therefore, time taken to spread the rumour to 729 persons is 30 minutes.