How do you work out one third plus one fifth? - Answers

To determine the sum of two fractions with different denominators, you must first make the denominators the same. One way to do this is to multiply the denominators by each other to get the new, equal denominator, and then cross multiple the old denominators with the numerators to get equivalent fractions. Sound hard? Really, it's not! Take 1/3 and 1/5. Cross multiply the top and bottom. 5*1=5 and 3*1=3. So your new numerators are 3 and 5. Now multiply the denominators. 3*5=15. Now your new fractions are 3/15 and 5/15. These are the same as 1/3 and 1/5, they are just not simplified. Now you can just add the numerators, keep the same denominator, see if you can simplify, and you're done! 3/15+5/15=8/15, which is can't be reduced any further.