Anita is 4 years older than her brother Rohit, and Rohit is 3 years older than their cousin Sameer. The three ages add up to 43 years. How old is Rohit?

Write everything against Rohit, $r$: Anita is $r+4$ and Sameer is $r-3$, so $r+(r+4)+(r-3)=3r+1=43$ and $r=14$. Check: 18 + 14 + 11 = 43. 12 comes from $3r+7=43$, adding both gaps as though Sameer were also OLDER than Rohit; one gap goes up and the other goes down, so they nearly cancel. 18 and 11 are Anita's and Sameer's ages - correct numbers, wrong person.