Does "mass of flour left" mean "mass of remaining flour" ?

Question

Betty had 3/4 kg of flour. She used 1/4 of it to make a cake. What was the mass of flour left in grams?

Answer 1

This is an example of a mathematical word problem that uses a term (left) that can be confused. Students may read the word "left" and think the term refers to direction. However, in this case, left does not mean the opposite of right, it means remaining.

To solve the problem, we must first take the original amount of flour which is 3/4 kg, and subtract the 1/4 kg of flour used. To subtract fractions when the denominator (the bottom number) is the same as in our problem (4), we can subtract across the numerators (the top numbers). Thus, 3-1 = 2, and the new fraction becomes 2/4 kg.

Whenever possible, fractions should be simplified. Since 2 goes into 4 evenly, we divide the numerator and denominator of our fraction by 2 and get the simplified result of 1/2 kg.

We know that we have 1/2 kg, but that doesn't help us solve the problem unless we change from a fraction to a number and also convert from kilograms to grams (the problem asks for the amount in grams).

To change from a fraction into a number, we take the fraction 1/2 and divide the numerator (1) by the denominator (2) and we get the result of .5 kg.

Now we can convert from kilograms to grams by remembering that 1 kilogram is worth 1000 grams. Thus we take .5 (the number of kilograms we have) and multiply by 1000 (the number of grams in a kilogram). .5 x 1000 = 500. Therefore we have 500 grams.

Final answer properly expressed:

There are 500 grams of flour left.

Note that I gave my response in sentence form and still used the term "left" because that is the word choice of the problem.