Question

how long does it take to transmit x kb over y mbps

Answer 1

Data volumes are usually measured in Bytes (B) whereas speed of channel is usually measured in bits(b).

1 B = 8 b
1 KB = 1024 B , 1 kb = 1024 b
1 MB = 1024 KB , 1 mb = 1024 kb

In order to calculate the time taken, convert the data volume and speed into same units and then divide volume by speed.
$data \: = \: \frac{volume}{speed}$

So if you say the data volume is x KB, then it would take a time of in seconds,
$\frac{x \times 8}{y \times 1024}$
And if you say (as per the question) x kb, then it would take a time of in seconds,
$\frac{x}{y \times 1024}$