I have a set of data (mass measurements from an 8th grade science lab) and the students took "Before" and "After" measurements. They calculated the difference in mass (for example, 21.45 g is before; 21.43 g is after, the difference should be -0.02 g) using Excel. They are supposed to generate a histogram - but the results are incorrect because Excel uses 15 significant digits and instead of returning a value of -0.02 g, Excel returns a value of -0.199999999999996 g. Why does this happen and how do I easily correct this problem? The histograms are turning out wrong because these minute decimal place errors are causing numbers to be placed in the incorrect bin.