The three devices you are going to install are a 2u server, a 1u keyboard tray, and an 8u space for a rack mounted moitor. given that and the existing components in the rack:

what size rack will you need to house everything?

Better to have 25 units racks is best to fix 2 servers 1 keyboard and monitor.

Explanation:

Server racks are measured in the unit. Each unit has a 1-inch size in height. Where end-user planning to install the following things

1. Number of the server is 2; each server height is 3 units.

Details as follows

One server height is 3 units

The total number of servers is 2. Therefore 2 servers = 6 units.

The gap between servers is 2 units.

The Gap above server is 1 unit

The gap below server is 1 unit

So total number of units = 6+2+1+1 = 10 unit

2. keyboard where it has 1 unit.

The gap between sever and keyboard 1 unit

So total number unit is 1+1 = 2 units

3. monitor  has 8 units

The gap above monitor and below monitor 1 unit  and after 1 unit

So total number units is 8+1+1 = 10 units

As conclusion,  

 Total No. of the unit for the server is = 10 units

 Total No. of the unit for the keyboard is = 2 units

 Total No of the unit for the monitor is   = 10 units

 Total no.of units requires = 22 units

Better to have 25 units are idle.