Doubt in ray optics

I am getting a different answer !
Where you not able to solve it or you also got a different answer than what it’s given over there ?

Even I got a different answer .I have its solution but I am not able to understand last part. Pls see it and make me understand last part

The portion where where they calculated t2. Why they have calculated t2 isn't t1 asked in question

I did a calculation mistake , the answer is correct .
I’ll post complete solution if you’ll need.

For now , t2 is subtracted from t1 because , the max height that the man can reach above the point of it’s projection is20m but it can only see the image till 10m above its point of projection , and so we it see the image when it goes fron 0-10m then he can’t see any image from 10m-20m-10m and then again from 10m to -12m it can see the image
Here i have considered point of projection to be origin

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Thanks @Shwetanshu_2018 .I have done a blunder in understanding question and then its solution also.