Some Glutton!

I was lunching in a South India restaurant. The place was crowded. A man excused himself and sat at my table. 

He began to eat idlis one after the other. As soon as one plate was finished he ordered more. As I sat there discreetly watching him, somewhat stunned, after he finished the last idli he told the waiter that he did not want any more.

  He took a big gulp of water, looked at me, smiled and said ‘The last one I ate was the 100^th idli in the last five days. Each day late 6 more than on the previous day. Can you tell me how many I ate yesterday?’

Answer: He ate 26 idlis on the fourth day.

Day                         No. Eaten

1^st day                          ×

2^nd day                   ×   +   6

3^rd day                   ×   + 12

4^th day                   ×   + 18

5^th day                   ×   + 24

Total                   5 × +60 = 100;   × = 8; 8+18 = 26