at what time will it have upwards velocity of u/4

I have question a level physics revision workbook. That doesn't make any sense to me:

A projectile is fired vertically upwards with an initial velocity u at time t it has upwards velocity of u/2 at what time will it have upwards velocity u/4.

I checked the answers at the back but it is not clear it says the answer is 3/2t.

But there is not much working and most of explaining where they get the numbers from is skipped. Can someone explain this question to me and how to get an answer?

It just uses
v = u + at
Every t/2, the speed reduces by u/4, as the relationship is linear.

The velocity will reduce by u/2 every t.

I have question a level physics revision workbook. That doesn't make any sense to me:
A projectile is fired vertically upwards with an initial velocity u at time t it has upwards velocity of u/2 at what time will it have upwards velocity u/4.
I checked the answers at the back but it is not clear it says the answer is 3/2t.
But there is not much working and most of explaining where they get the numbers from is skipped. Can someone explain this question to me and how to get an answer?