Length of rod given forces and lengths to calculate torque [closed]

In the question and solution given below, I haven't understood why is the solution asking us to take torques about the center of mass.
In my approach I equated the net torque about the lower end B of the rod to zero

MY APPROACH:
let distance between line of action of F2 and B = k cm
then length of the rod = k+20 cm
taking torques about B, we have
F2k - F1(k+20) = 0 (since rod is only translating)
Putting F2 as 5N , and F1 as 3N, we have k=30cm
thus my answer comes out as 50cm

*ps : we cant take F1 as 7N. I already tried that. torque being zero is impossible in that case

• physics.stackexchange.com/q/93698 I hope this helps
– S.M.
Apr 29, 2021 at 4:05
• Hello and welcome to the physics stack exchange! Since this is not a typical help site, it would be great to ask questions to benefit the wider audience. Apr 29, 2021 at 4:19
• You can refer to this link for more details : How to ask homework type questions on this site. In case you find these rules to be a bit harsh, you can always go to any other suitable sites. As someone said before, you can learn the alphabet from these sites and then come back here to ask questions about dinosaurs :) Have a great time! Apr 29, 2021 at 4:19
• @AlphaLife this isn't my home work question. i am not able to understand the concept of torque behind it.....as in, which point to take and why Apr 29, 2021 at 6:45
• @S.M. tnx. it helped Apr 29, 2021 at 6:53

Let $$A$$ and $$B$$ be two point on some body. Let $$\vec \tau$$ denote the torque, and $$\Sigma \vec F$$ be the net force. Then, the torque about A is related to the torque about point B by the following equation $$\vec \tau_B = \vec \tau _A + \vec r_{AB} \times \Sigma \vec F$$