A snail is climbing a clock tower which is 45 metres high.~ It climbs up 10 metres during the day and slides down during the night. 1 metre on the 1st night, 2 metres on the 2nd night,~ 3 metres on the 3rd night, $\cdots$ and so on. Which day would reach the top of the tower? 
Overseas Competition->Knowledge Point->Word Problem Modules->Periodic Problems->Snail Climbing out of Well Problems