The M-% (calc-percent) command takes a percentage value,
say 5.4, and converts it to an equivalent actual number. For example,
5.4 M-% enters 0.054 on the stack. (That's the META or
ESC key combined with %.)
Actually, M-% creates a formula of the form `5.4%'.
You can enter `5.4%' yourself during algebraic entry. The
`%' operator simply means, "the preceding value divided by
100." The `%' operator has very high precedence, so that
`1+8%' is interpreted as `1+(8%)', not as `(1+8)%'.
(The `%' operator is just a postfix notation for the
percent function, just like `20!' is the notation for
`fact(20)', or twenty-factorial.)
The formula `5.4%' would normally evaluate immediately to 0.054, but the M-% command suppresses evaluation as it puts the formula onto the stack. However, the next Calc command that uses the formula `5.4%' will evaluate it as its first step. The net effect is that you get to look at `5.4%' on the stack, but Calc commands see it as `0.054', which is what they expect.
In particular, `5.4%' and `0.054' are suitable values for the rate arguments of the various financial functions, but the number `5.4' is probably not suitable--it represents a rate of 540 percent!
The key sequence M-% * effectively means "percent-of." For example, 68 RET 25 M-% * computes 17, which is 25% of 68 (and also 68% of 25, which comes out to the same thing).
The c % (calc-convert-percent) command converts the
value on the top of the stack from numeric to percentage form.
For example, if 0.08 is on the stack, c % converts it to
`8%'. The quantity is the same, it's just represented
differently. (Contrast this with M-%, which would convert
this number to `0.08%'.) The = key is a convenient way
to convert a formula like `8%' back to numeric form, 0.08.
To compute what percentage one quantity is of another quantity, use / c %. For example, 17 RET 68 / c % displays `25%'.
The b % (calc-percent-change) [relch] command
calculates the percentage change from one number to another.
For example, 40 RET 50 b % produces the answer `25%',
since 50 is 25% larger than 40. A negative result represents a
decrease: 50 RET 40 b % produces `-20%', since 40 is
20% smaller than 50. (The answers are different in magnitude
because, in the first case, we're increasing by 25% of 40, but
in the second case, we're decreasing by 20% of 50.) The effect
of 40 RET 50 b % is to compute (50-40)/40, converting
the answer to percentage form as if by c %.