expected expenditure

[hs]

Q: Envelopes are on sale for Rs 30 each. Each envelope contains exaclty one coupon, which can be one of the four type with equal probability. suppose you buying envelopes and stop when you collect all the four types of coupons. What will be your expected expenditure?

[Gayyam]

Here is Amit sir's solution.

[hs]

a guy taking goel sir classes asked me this lol
may b he was mocking me

[Gayyam]

Haha! Irony!

[SoniaKapoor]

What a question!