With what acceleration 'a' should a box descend so that a block of mass M placed in it exerts force Mg/4 on the floor of the box?

Explanation

The block is at rest with respect to the box which is accelerating with respect to the ground. So, the acceleration of the block w.r.t. the ground is .a. downward. The forces on the block are as follows:
i) Weight Mg downward (exerted by the earth)
ii) Normal reaction R upward (exerted by the floor)
Equation of motion of block is:
`Mg- R = Ma , " But " R=(Mg)/(4)`
`:. Mg-(Mg)/(4) =Ma or (3Mg)/(4) =Ma implies a=(3g)/(4)`
The block and hence the box should descend with an acceleration 3g/4.