Add and subtract decimals or fractions: word problems

Learn with an example

💣 While making desserts for a bake sale, Zach used 1/4 of a scoop of brown sugar as well as 1/6 of a scoop of white sugar. How much more brown sugar did Zach use?

💣 Write your answer as a fraction or as a whole or mixed number.

______scoops

  • Find how much more brown sugar was used by subtracting 1/6 from 1/4.
  • To subtract the fractions, you can use the lowest common denominator. The lowest common denominator is the smallest whole number that is a multiple of both 4 and 6.
  • Write the prime factorisation for each denominator.
  • 4 = 2 × 2
    6 = 2 × 3
  • Repeat each prime factor the most number of times it appears in any of the prime factorisations above.
  • The most the factor 2 appears is twice.
  • The most the factor 3 appears is once.
  • So, multiply:
  • 2 × 2 × 3 = 12
  • The lowest common denominator is 12. Find equivalent fractions with 12 as the denominator.
  • 1 × 3/4 × 3 =3/12
  • 1 × 2/6 × 2 =2/12
  • Now subtract the fractions. Subtract the numerators and keep the denominators the same.
  • 3/12−2/12=1/12
  • Zach used 1/12 of a scoop more brown sugar.

💣 A carpenter bought a piece of wood that was 4.6 centimetres long. Then she sawed 0.5 centimetres off the end.

How long is the piece of wood now?

_____ centimetres

  • Subtract the numbers of centimetres. Remember to line up the decimal points.
  • 4 . 6 – 0 . 5= 4 . 1
  • The piece of wood is 4.1 centimetres long now.

💣 Of the shirts in Savannah’s chest of drawers, 7/10 are striped, while another 1/5 are plaid. What fraction of the shirts are either striped or plaid?

💣 Write your answer as a fraction or as a whole or mixed number.

_______of the shirts

  • Find the fraction of the shirts that are either striped or plaid by adding 7/10 and 1/5.
  • You can use 10 as the common denominator.
    Multiply the numerator and the denominator of
    1/5
    by 2 to find an equivalent fraction.
  • 1 × 2/ 5 × 2 = 2/10
  • Now add the fractions. Add the numerators and keep the denominators the same.
  • 7/10+2/10 = 9/10
  • The fraction of the shirts that are either striped or plaid is 9/10.

Let’s practice!🖊️