excell miscalculation

[angela6704]

Excel is giving -7.4E-13 for 60-60. Can anyone explain this? These are the
values and formulas I have for each cell.

a1 = 25421.54
b2 = 23941.24
c1 = a1-b1 (1480.3)
d1 = 1016.86
d2 = 198
d3 = 205.44
d4 = 60
e1 = c1-d1 (463.44)
e2= e1-d2 (265.44)
e3 = e2-d3 (60)
e4 = e3-d4 (-7.4E-13) (60-60 s/b 0)

Thanks

 

[Jerry W. Lewis]

Excel (like almost all computer software) uses binary arithmetic. In binary,
most terminating decimal fractions are non-terminating binary fractions that
can only be approximated (just as 1/3 can only be approximated as a decimal
fraction). When your inputs are approximate, it should not be a surprise
when the output is also only approximate.

All of the arithmetic is correct here, given the initial approximations to
your decimal fractions. The impact of these approximations can actually be
seen much earlier; format E1:E3 to display 15 figures.

Since you are subtracting numbers with no more than 2 decimal places,
anything beyond the 2nd decimal place must be the residue of binary
approximations to the numbers, so if you round each result to 2 decimal
places, you will avoid surprises while doing no violence to the calculations.

Jerry