ISMv2 - Disk Service Time

i'm preparing for the exam and I'm finding the 2nd module challenging.  Specifically the topics of Disk Service Time, Utilization, etc.  I have access to the e-learning, the VILT and the book and I am missing the mathmatical principle that allows you to determine the data transfer time somehow.  If I take a value of size such as 4 kb, how is it that i can formulate a fraction placing 4 kb (size) over (/) 40 mb/s (rate) ?  Please consider it has been quite some time since I've practiced math like this but are you able to reduce this fraction to 1/10 because you are dealing with different unit types?  In other words, if I formulate a fraction of 4kb over 40mb (size not rate), then wouldn't i need to convert 40mb to it's equivalent in kb and reduce from there?

Thank you for clarifying!  Also, how much of the exam is focused on the equations for service time, utilization, etc? it seems that this is briefly covered in the teaching materials but I am afraid if this is a major focus on the exam it would be easy to get 'tripped up' and get questions marked wrong fairly easy.