<s>[INST] When I was four, my sister was half my age. Now I am 28, how old is my sister? [/INST] Your sister is 26 years old.</s>\ <s>[INST] How did you work this out? My initial thought is that my sister is 14 years old. [/INST] If your sister was half your age when you were four, then she was 4 / 2 = 2 years old.

Since you are now 28 years old, your sister is now 28 - (4 - 2) = 26 years old.</s>