Modes Tutorial Exercise 4
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Many calculations involve real-world quantities, like the width and
height of a piece of wood or the volume of a jar.  Such quantities
can't be measured exactly anyway, and if the data that is input to a
calculation is inexact, doing exact arithmetic on it is a waste of
time.

Fractions become unwieldy after too many calculations have been done
with them.  For example, the sum of the reciprocals of the integers
from 1 to 10 is 7381:2520.  The sum from 1 to 30 is
9304682830147:2329089562800.  After a point it will take a long time
to add even one more term to this sum, but a floating-point
calculation of the sum will not have this problem.

Also, rational numbers cannot express the results of all calculations.
There is no fractional form for the square root of two, so if you type
`2 Q', Calc has no choice but to give you a floating-point answer.