HP HP-67 - Page 73

The Automatic Memory Stack 69
Example: A bacteriologist tests a
certain strain whose population
typically increases by 15% each day.
If he starts a sample culture of 1000,
what will be the bacteria population at
the end of each day for six consecutive
days?
Method: Put the growth factor (1.15) in the Y-, Z-, and T-registers
and put the original population (1000) in the X-register. Thereafter,
you get the new population whenever you press £3 .
Press Display
1.15 Growth factor.
Growth factor now in T.
1000 1000. Starting population.
1150.00 Population after 15 day.
1322.50 Population after 2" day.
a 1520.88 Population after 3™ day.
8 1749.01 Population after 4 day.
8 2011.36 Population after 5™ day.
a8 2313.06 Population after 6' day.
When you press &4 the first time, you calculate 1.15 X 1000. The
result (1150.00) is displayed in the X-register and a new copy of the
growth factor drops into the Y-register. Since a new copy of the
growth factor is duplicated from the T-register each time the stack
drops, you never have to reenter it.
Notice that performing a two-number operation such as EJ causes
the number in the T-register to be duplicated there each time the
stack is dropped. However, the (R¥) key, since it rotates the contents of
the stack registers, does not rewrite any number, but merely shifts
the numbers that are already in the stack.