To get an exact answer for a test, posters have already showed the correct formula.

For in flight use what you really want is TLAR (That Looks About Right) skills.

This means using easy approximations to get a "good enough" number.

So in for your original question "how far will I go at 98 kts in 2 1/2 mins" I would do the quick mental calculation as follows.

-Round up or down to the nearest 1/2 mile per min ( ie 60 kts = 1 mile/min, 90 kts = 1 1/2 mile/min 120 kts = 2 mile/min. Therefore 98 kts is closer to 90 than 120 so use 1 1/2 mile/min value

- calculate the whole numbers first than add the remainder. so 2 times 1.5 = 3 miles plus half of 1.5 ( for the remaining 1/2 min )= 3/4 gives you 3 + 3/4 = 4 1/4 miles an answer that is close enough.

You can do this in your head in maybe 2 seconds. For in flight calculations it will be rare that a calculation of this kind will not be good enough. If you need more accuracy you should use a Whiz Wheel but again a mental TLAR calculation is a good check.