MathChapter 6: Ratios, Rates, and Proportions
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Prompt
What is a ratio?
💡The category named first occupies the first ratio position.
Answer
A comparison of two quantities by division in a specified order.
Example\(12\) blue to \(8\) gold is \(12:8=3:2\).
Prompt
What three notations can express the ratio of \(a\) to \(b\)?
💡Each notation preserves the same order.
Answer
\(a\) to \(b\), \(a:b\), and \(a/b\).
Example\(3\) to \(5\), \(3:5\), and \(3/5\) are equivalent.
Prompt
How are quantities in ratio \(a:b\) represented with a common scale factor?
💡Both categories must scale together.
Answer
As \(ax\) and \(bx\) for the same positive scale factor \(x\).
ExampleA \(2:7\) ratio can produce \(6\) and \(21\) when \(x=3\).
Prompt
What is a rate?
💡Look for a compound unit joined by ‘per.’
Answer
A ratio comparing measurements with different units.
Example\(72\) kilometers per hour is a rate.
Prompt
What is a unit rate?
💡Normalize the denominator quantity.
Answer
A rate whose denominator is \(1\) unit.
Example\(24\) pages per \(1\) minute is a unit rate.
Prompt
How is unit price calculated?
💡The requested unit is dollars per item.
Answer
Divide package price by the number of units in the package.
Example\(\$14/20=\$0.70\) per item.
Prompt
How is gas mileage in miles per gallon calculated?
💡Miles belong above gallons.
Answer
Divide miles traveled by gallons of fuel used.
Example\(360/12=30\) miles per gallon.
Prompt
How is speed calculated from distance and time?
💡Use compatible distance and time units.
Answer
\(\text{speed}=\text{distance}/\text{time}\).
Example\(150\) kilometers in \(3\) hours is \(50\) kilometers per hour.
Prompt
How is density calculated?
💡Mass belongs in the numerator.
Answer
\(\text{density}=\text{mass}/\text{volume}\).
Example\(240\) grams over \(20\) cubic centimeters is \(12\) grams per cubic centimeter.
Prompt
Why does numerator-denominator order matter in a rate?
💡Miles per gallon is not gallons per mile.
Answer
Reversing the order creates the reciprocal rate and answers a different question.
Example\(30\) miles per gallon has reciprocal \(1/30\) gallon per mile.
Prompt
How do you distribute a known total through ratio \(a:b:c\)?
💡Add every ratio part before finding \(x\).
Answer
Represent the categories as \(ax\), \(bx\), and \(cx\), then solve \((a+b+c)x=\text{total}\).
ExampleFor \(2:3:5\) totaling \(100\), one part is \(10\).
Prompt
What mistake occurs when a total is treated as one ratio part?
💡Use the sum of the ratio coefficients.
Answer
Every category becomes too large because the total represents the sum of all scaled parts.
ExampleFor \(3:4\) totaling \(56\), solve \(7x=56\), not \(3x=56\).
Prompt
How do you simplify a ratio?
💡Do not divide only one position.
Answer
Divide every part by the same greatest common factor.
Example\(24:36\) simplifies to \(2:3\).
Prompt
How can you check two ratios for equivalence without decimals?
💡One multiplier must transform the entire ratio.
Answer
Compare cross products or verify that both parts use the same scale factor.
Example\(4:7\) and \(20:35\) both use factor \(5\).
Prompt
In a triangle-angle ratio problem, what total should the scaled angle expressions equal?
💡Use the angle sum of a triangle.
Answer
\(180^\circ\).
ExampleFor \(2:3:4\), solve \(2x+3x+4x=180\).
Prompt
How should mixed measurement units be handled before calculating a rate?
💡A compound unit cannot mix unmatched time scales silently.
Answer
Convert them to compatible units, then divide in the requested order.
ExampleConvert \(1.5\) hours to \(90\) minutes before finding pages per minute.
Prompt
What is the best first step when a question asks for ‘items per dollar’?
💡The wording specifies the fraction's order.
Answer
Write items above dollars before inserting the values.
Example\(12\) items for \(\$3\) gives \(12/3=4\) items per dollar.
Prompt
How can a relation such as \(5p=3q\) be converted to ratio \(p:q\)?
💡Coefficients move to the opposite ratio positions.
Answer
Isolate \(p/q\): \(p/q=3/5\), so \(p:q=3:5\).
ExampleDividing \(5p=3q\) by \(5q\) gives \(p/q=3/5\).
Prompt
How do you solve a ratio problem when the difference between categories is known?
💡The coefficient difference represents the stated difference.
Answer
Subtract the scaled expressions, solve for the common multiplier, then find the requested amount.
ExampleFor \(7x-3x=20\), \(x=5\).
Prompt
What final checks should a ratio or rate answer pass?
💡Audit meaning as well as arithmetic.
Answer
Correct order, simplified value, compatible units, and agreement with the original total or measurements.
ExampleA mileage answer should be labeled miles per gallon and reproduce the total miles when multiplied by gallons.
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