



Sometimes
it would be nice if we had a quick way 


to
get a number that was very close to the right answer. 





WE
DO! 





This
is a trick that is mostly used with multiplying, 


but
can really be used anywhere. 


Say
you want to know about how much you get 


when
you multiply 48 times 32. 











We
COULD work this out, 


but
watch this. 


48
is close to 50. 


32
is close to 30. 


If
we multiplied 50 times 30 we would get a number 


that
was close to the answer
of 48 times 32. 





That's
pretty good, but we can make it easier. 





50
is just a 5 in the tens column. 


30
is just a 3 in the tens column. 





If
we multiply 5 times 3 we will get about the right number 


except
for the number of zeros we need in the answer. 





5
x 3 = 15 





Now
what about those zeros? 





Here's
how it works. 


50 has one
zero.



30
has one zero. 


That's
2 zeros, 


and
that's how many go in the answer. 





So
our guess is 1500. 





The
real answer when we multiply 48 times 32 is 1536. 


Our
guess was off by less than 3 percent. 





Try
another one ... 





Example: 











615
is close to 600 


491
is close to 500 





6 x 5 = 30 





The
numbers had a total of 4 zeros in them 


so
add 4 zeros to the 30. 


You
get: 


300,000 





The
real answer to 615 x 491 is 301,965. 


Not
to bad, eh? 





If
you need to solve any problem, 


you
should go ahead and use this trick first. 





Then
you will know about what the answer should be. 


If
you get something that's WAY
different, 


you
will know that you made a mistake somewhere. 


That
type of mistake happens a lot when people use calculators. 


Calculators
don't make mistakes when they do math. 


But
if you hit a wrong key, 


the
calculator will very nicely calculate the wrong answer. 





copyright 2005 Bruce Kirkpatrick 
