What is the mass of a double pulley if.....????

Question

The total mass of the LOAD is 300
The total mass of EFFORT is 75

Answer 1

Hmmm... I wish I could see what yo are talking about.
Depending on the double puley configurations we can have 3 or 2 advantage. So unless you puley has a negative mass of -25 or -75 what you are asking is impossible.

On the other hand if you have double tandem pulley where advantage is 4 then the lower pulley is massless.

Continuing along these lines we can have a triple block of of double pulley with a atotal effort of 6. Then

(300 + m)/6=75
m= 75 x 6 - 300= 150
That is a heavy pulley!

I hope that helps.