How fraction arithmetic works
To add or subtract fractions they must share a denominator. The calculator uses the least common multiple of the two denominators, rewrites each fraction over it, then adds or subtracts the numerators. Multiplication multiplies numerators and denominators straight across; division multiplies the first fraction by the reciprocal of the second.
Every result is reduced to lowest terms by dividing the numerator and denominator by their greatest common divisor, and the sign is kept on the numerator. If the result is top-heavy it is also shown as a mixed number and as a decimal.