HP 20B - Manual (Page 27)

Mathematical Calculations
27
 nCr
 nPr
Y x is the power function, nCr stands for the number of combinations of n items taken r at a time, and nPr stands for the
number of permutations of n items taken r at a time.
Combination=
 
! !
!
r n r
n

Permutation=
 
!
!
r n
n

Perform calculations with these functions in the same way you would perform calculations with
+-/
and
*
,
but press
:
to access the secondary function key. For example, to calculate 15 3 :
1.
Press
15
.
2.
Press
:h
.
3. Press
3=
.
The results are shown Figure 9.
Figure 9
In RPN mode, key in the numbers first, followed by
I
, then press the function key. For example, for the power function
example above, in RPN press:
15I3:h
.
Storing and Recalling Numbers
The calculator has ten memories available for use during calculations. These memories are numbered from 1-9 and 0. To
store a number in a memory, press
:$
, followed by the key representing the memory number. To recall a number
stored in a memory, press
L
followed by the key representing the memory number. You can use the store and recall
functions for these memories any time a number is displayed, or when you wish to enter a number.
For example, to store 15 in memory 1, press
15:$1
.
To recall the number in memory 1, press
L1
.
You can also perform operations to numbers stored in memories. For example, press
5:$2
to store 5 in
memory 2. To add 12 to the value of memory 2, press
12:$+2
. Later on, during a calculation, you can