Carpentry & Building Construction © 2004

Dividing Fractions

Carpenters need to know how to divide fractions for some calculations. For example, carpenters may have to divide fractions when laying out stairs or deciding how much material was used.

To divide fractions, begin by changing mixed numbers to improper fractions. Next, use the rule of keep-change-flip. Keep the first fraction, change the sign from division to multiplication, and then flip (invert) the second fraction. To complete the operation, reduce numerators and denominators to lowest terms. Multiply the numerators and the denominators. Convert the improper fraction to a mixed number. Check the number for lowest terms.

 Practice Exercise

Divide the mixed number 10 1/2 by 2 3/4.
Step 1	Change both numbers to improper fractions.
	10 1/2 becomes 21/2
	2 3/4 becomes 11/4
	Now you have 21/2 ÷ 11/4
Step 2	Keep the first fraction. Change the division sign to multiplication. Flip the second number.
	21/2 × 4/11
Step 3	Multiply the numerators and denominators.
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Step 4	Change 84/22 to a mixed number.
	84/22 = 3 18/22
Step 5	Put in lowest terms.
	3 18/22 = 3 9/11

Problem Exercises

1.		Divide 8 1/4 by 3 1/2.
			A)	2 1/8
			B)	2 5/14
			C)	24 1/8
			D)	28 7/8
2.		Divide 9 1/2 by 2 3/4.
			A)	3 5/11
			B)	3 5/8
			C)	4 3/8
			D)	4 5/11
3.		Find the height of a riser when the total rise of a set of stairs is 109 1/2'' and there are 15 risers.
			A)	7''
			B)	7 3/10''
			C)	7 1/2''
			D)	7 2/15''
4.		An installer is tiling a bathroom wall using 4'' tiles with a 1/8'' mortar joint between rows. So far the tiled wall is 49 3/8'' high. How many tiled rows were installed? Need a Hint?
			A)	11
			B)	11 1/2
			C)	11 7/8
			D)	12
5.		Find the width of each tread for a set of stairs when the total run is 146 1/4'' and there are 13 treads.
			A)	11
			B)	11 1/8
			C)	11 1/4
			D)	11 3/8