Add two fractions and reduce the result.
First give both fractions a common denominator, then add the numerators. For 1/3 + 1/4 the common denominator is 12, so 4/12 + 3/12 = 7/12. Multiplying the denominators together always works, but the least common multiple keeps the numbers smaller and leaves less cancelling to do at the end.
Replace the division with multiplication by the reciprocal: turn the second fraction upside down and multiply. So 2/3 ÷ 4/5 = 2/3 × 5/4 = 10/12 = 5/6. This works because dividing by a number is the same as multiplying by its reciprocal.
Divide the numerator and the denominator by their greatest common divisor. For 18/24 the greatest common divisor is 6, so 18/24 = 3/4. If the greatest common divisor is 1, the fraction is already in lowest terms. Simplifying never changes a fraction's value, only the size of the numbers writing it.
Turn a fraction into a decimal by dividing the numerator by the denominator: 3/8 = 0.375. Going back, write the decimal over 10, 100 or 1000 and simplify: 0.375 = 375/1000 = 3/8. Fractions whose denominators carry prime factors other than 2 and 5 give a repeating decimal — 1/3 = 0.333…