On which day of the month is his birthday?

Question

As Mohan's birthday approaches , he likes to save up to to treat himself.
On the first day of the month he saves Rs 1, on the second day he saves 2, on the third day 3 and so on.
On his birthday he will have saved a total Rs 153.
On which day of the month is his birthday?

Answer 1

18

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Days- x,
Sum of x days calculated by a formula: Sx=x*(x+1)/2,
x*(x+1)/2=153,
The positive solution is x=18