Using Newton's second law
\(mg - T = ma _{1}\)
\(500- T =50 \times 4 \Rightarrow T =300\,N\)
F.B.D of monkey while moving up
Using Newton's second law of motion
\(T - mg = ma _{2}\)
\(T -500=50 \times 5\)
\(T =750\,N\)
Breaking strength of string \(=350\,N\)
String will break while monkey is moving upward