Question

calculate the scale reading when the elevator descends with a constant velocity?

Answer 1

You should post the whole question

If you are talking about a weighing scale - then when moving with constant velocity it will indicate the same as when at rest.

If the lift is accelerating then it will change the scale reading.

(this is assuming it is a 'spring balance' type scale. If it is a balance beam type then it will not change under any linear motion.

(this is because spring balance measrures weight, but balance beam measures mass.