I used to play these games all day back in high school. trust me i have had enough subs thrown my way to answr any riddle. this is your answer:
Flip the 7 and 11 min clocks at the same time because once the 7 mins runs out out the first clock, that tells you that there are only 4 mins remaining in the 11 minute clock. once the 7 clock runs out throw it away and immediatly flip the 11 minute one back over for the full eleven minutes. 4+11=15
elementary my dear watson
Just round it. 9 rounds up, causing the 9 in front to become 10, causing the 9 in front to again become 10, and so on until it rounds out to 1. Anyway, could I post some of my own? They're not math problems, but they're still riddles.
There is a town that wishes to expand their fountain to twice it's size, but there's a problem. The fountain is square, and arranged with a tree at each corner. The town doesn't want to pay to have the trees removed,so how can they double the fountain's size, while removing the least amount of trees?