I get this problem of the app Tsumego pro. It is problem number 110 of Daily problems-hard. Black must kill.
If black play in R15, white play S15. After (may be revesed) black play Q18 and white P17. And the problem finish with R16 for black.
It isn't dificult but it is some stupid. Has it a good approach? I got this doubt because if white sacrificing some rocks, white can live. For example, if black do R15 white answers R16 and he can live. If black do Q18, white can answer R15 and he live.
Why is that? Are they misguided tsumegos with some answers to get all or nothing?
When I say tsumegos I don't want say that all of this app, but there are some in internet and other sites that I think that they are making to get all or nothing.
Sorry for no make a good diagram.