Solving riddle!

Yeah,so here i come to solve yin's 4th riddle.As usual i quite lazy to write leh..But solve riddle still okok..hehe..ok la.I'll write next time.Now quite late le.Aniwaes,is the answer to this riddle logical or cheeky one?If it's logical,I'll offer the following solution:


Use both hourglasses.
Let both 4min-hourglass n 7min-hourglass run till the 4min-one finish.Immed flip over the 4min-hourglass.We hav account for 4 mins here(left 3 min in 7min-hg).
Now,let both run agn till 7min-one finish.Immed flip over the 7min-hourglass.We hav acct for 3mins here(left 1 min in 4min-hg).
Let both run agn till 4min-one finish.Immed flip over the 7min-hourglass.we hav acct for 1 min here(left 1 min in 7min-hg).
Finally,let the 7-mind hourglass runs out of the 1 min.

Thus,we get 9mins,neglecting the time we took to flip the hourglasses.tio bo?

[L]uN