Adding or subtracting fractions can be done quickly by utilizing some of multiplication techniques discussed in earlier blogs.
Example:
A particular cookie recipe called for 3/4ths of a cup of sugar. When the baker wasn't looking a young child added multiple sugar packets to the dough, approximately 2/5ths of a cup worth of sugar. How much total sugar was there in the dough?
Example illustrated:
3 2
-- + -- = ?
4 5
Step 1) Multiply diagonally to get the numerator. So 3 * 5 and 4 * 2
15 + 8 = 23
Add the two products, 15 and 8, together because it is an addition problem. If however the problem was subtracting fractions we would subtract the products instead to get the numerator.
Step 2) To get the denominator multiply the two denominators together. So 4 * 5
4 * 5 = 20
Step 3) The sum of 3/4 and 2/5 is 23/20, reduce as necessary.
Answer: 1 and 3/20ths cups of sugar (1.15)
Example:
A particular cookie recipe called for 3/4ths of a cup of sugar. When the baker wasn't looking a young child added multiple sugar packets to the dough, approximately 2/5ths of a cup worth of sugar. How much total sugar was there in the dough?
Example illustrated:
3 2
-- + -- = ?
4 5
Step 1) Multiply diagonally to get the numerator. So 3 * 5 and 4 * 2
15 + 8 = 23
Add the two products, 15 and 8, together because it is an addition problem. If however the problem was subtracting fractions we would subtract the products instead to get the numerator.
Step 2) To get the denominator multiply the two denominators together. So 4 * 5
4 * 5 = 20
Step 3) The sum of 3/4 and 2/5 is 23/20, reduce as necessary.
Answer: 1 and 3/20ths cups of sugar (1.15)
