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A recursive solution: number the rows $0$ to $r-1$ with $0$ being the fattest, notice that the difference between consecutive numbers in row $k$ is $2^k$.
Denote by $f(n)$ the number in the last row of a triangle with $n$ rows. Then $f(n)=2f(n-1)+2^{n-2}$
Notice $f(1)=0,f(2)=1,f(3)=4$
From th...