Another calculating tip. After the second multiplication, you can
almost always say the millions part of the answer (here, 33 million)
and hold the hundreds digit on your fingers. Here, just raise 1 finger
to “hold onto” the hundreds digit so you only have to remember 686
while you calculate the cube of 23. By the way, if you just want a good
cubing approximation, simply compute z(z(z + 3d)).
For example,