Cross Multiply and Divide!

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Presentation transcript:

Cross Multiply and Divide! Percentages Cross Multiply and Divide!

Bump, Set, Spike! Remember the volleyball player? Get everything set up, and the problems nearly solve themselves!

Set up! This is great, because if you know how to set it up, you can solve for any unknown! ___Is ___ = ___%___ Of 100

Unknown is the percent: Set it up! ___Is ___ = ___%___ Of 100 ___40___= n___ 120 100 (I know 40 is 1/3 of 120, so I’m going to simplify it to 1/3 =n/100. I also know that 1/3 = 33.% Forty out of 120 students at North Coventry came to make donations for make a difference day. What percent of students attended? Explain that you don’ t have any decimals to move in this way, because whatever you get is over 100, and that tells you the percent!

Unknown is the part of the whole: What is 25% or 36? n = 25 36 100 Now—cross multiply and divide. The two “knowns” crossed are 25 and 36. Mulitply those together. Then divide by 100 25X36=900. Now divide the product (900) by 100—the other “known.” 900 divided by 100= 9. So Daniel saved 9 dollars. He got a good deal! Daniel bought a Phillies shirt at 25% off. The original price of the shirt was $ 36.00. How much money did he save on the shirt? Note: You may notice that 25/100 is one fourth. Since we’re not looking at a percent but another aspect, you can simplify, and you will still get the correct answer with cross multiplication and division!

___Is ___ = ___%___ Of 100 28 = 15 n 100 Unknown is the whole ___Is ___ = ___%___ Of 100 28 = 15 n 100 Cross multiply: 28 X100 = 2800. Divide 2800 divided by 15 = 186.1 (186) So the total is 186 miles. The answer to the question , then is that they have 158 more miles to go. North Coventry 6th graders left for a field trip. After 28 miles, the teacher let the school know how far they had gone. The secretary told them they only drove 15 percent of the way. How many more miles must they travel? On this one, please note: solving for n did not get us the answer to the question. It did, however get us closer. Also again, you could have simplified 15/100 to 3/20 to make the cross mulitiplication and division easier!