Read 3 5/6 in the context of the inputs, rather than as a stand-alone fact. Write down the units and the meaning of each value if the result will be shared or used later.
The page first converts each mixed number to an improper fraction. It then combines the fractions and reduces the remaining fractional part with a greatest common divisor.
A denominator cannot be zero. Division also fails when the entire second mixed number equals zero, because no number can be divided by zero.
A calculator can perform consistent arithmetic, but it cannot judge whether a measurement, quote, survey or financial assumption is suitable. Check the source information before acting on the answer.
It is often helpful to run a second reasonable case. Changing one input at a time shows which assumption drives the result and gives a clearer picture than adding more decimal places to one estimate.
Keep rounding in proportion to the inputs. Exact conversion factors may produce long decimals, while measured and quoted values are usually less precise. Retain extra digits during the calculation and round at the end.
If another source gives a different figure, compare definitions before deciding that either answer is wrong. Similar names can hide a different unit, timing rule, standard, denominator or list of included costs.
For this mixed number calculator, a good written record contains the formula improper numerator = |whole| × denominator + numerator; then apply the selected operation and simplify, the entered values and the final rounding. That short note makes the result much easier to check later than a copied answer on its own.
