Answer
$T=\dfrac {2Mm}{M+mg}=\dfrac {2\times 15\times 10}{15+10}g=12g\approx 117.6N$
Work Step by Step
Let's assume that object is moving downwards with an acceleration a so monkey will be moving upwards with an acceleration $a$ Because length of the rope doesn't change Lets write Newton's Second Law for both monkey and object Assuming mass of the monkey is $m$ $ T-mg=ma\left( 1\right) $ Assuming mass of the object is $M$ $Mg-T=Ma(2)$ From (1) and (2) we get $T=\dfrac {2Mm}{M+mg}=\dfrac {2\times 15\times 10}{15+10}g=12g\approx 117.6N$
