Given:
The block and tackle system consists of 4
pulleys, i.e. n=4
In a n-pulley block and tackle system, if the effort is applied in the downward direction then the tension in the n segments of the string supports the load.
L=nT and E=T
Hence, Mechanical advantage
M.A.=loadLeffortE=nTT=n=4
Thus, the mechanical advantage is equal to the number of pulleys in both the blocks.
Given:
Load L=100N
M.A.=4
We know
M.A.=LoadLEffortE
Thus, the effort required,
E=LM.A.=100N4=25N