Acomputer's hard disk is spinning at 90 revolutions per second. through how many degrees does it travel in a second? in a minute?

Dimensional analysis 

90 rev / sec x 360 deg / rev = 32400 deg/sec 

32400 deg/sec x 60 sec/min = 

How many degrees are there in a circle? 360 degrees 

1 revolution = 1 full circle 

(360 degrees / 1 revolution) * (90 revolutions / 1 second) = 32,400 degrees / second 

same for the next part, just multiply by 60 to get degrees per minute

1 revolution means 360 degrees because a circle is 360 degrees

90 revolutions means 360 degrees times 90 revolutions=32400 degrees in a second

in a minute, we multiply by 60 because 60 seconds per minute

32400 times 60=1944000 degrees per minute