ON = AB = 3A'B'
\(\Delta OF'N\sim\Delta A'F'B'\)
\(\Rightarrow\frac{OF'}{A'F'}=\frac{ON}{A'B'}\Rightarrow OF'=\frac{10.AB}{A'B'}=30cm\)
=> OA' = OF' + A'F' = 40cm
\(\Delta OAB\sim\Delta OA'B'\)
\(\Rightarrow\frac{OA}{OA'}=\frac{AB}{A'B'}=3\Rightarrow OA=3OA'=120cm\)