MathChapter 5: Word Problems in Real-Life Situations
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Prompt
What relationship gives total value?
💡Check the units.
Answer
Number of items multiplied by value per item.
Example\(8\) items at \(\$6\) each have total value \(8(6)=48\) dollars.
Prompt
What is the profit formula?
💡Include fixed and variable costs.
Answer
\(\text{profit}=\text{revenue}-\text{total cost}\).
ExampleRevenue \(500\) minus cost \(320\) gives profit \(180\).
Prompt
How is revenue calculated for identical items?
💡Use sold quantity, not always purchased quantity.
Answer
Quantity sold multiplied by selling price per item.
ExampleIf \(x-4\) items sell for \(9\) dollars, revenue is \(9(x-4)\).
Prompt
What formula relates distance, rate, and time?
💡Distance is rate times time.
Answer
\(d=rt\).
ExampleAt \(50\) mph for \(3\) hours, distance is \(150\) miles.
Prompt
How do you solve \(d=rt\) for time?
💡Divide distance by rate.
Answer
\(t=d/r\).
ExampleTraveling \(120\) miles at \(40\) mph takes \(3\) hours.
Prompt
How do you solve \(d=rt\) for rate?
💡Divide distance by time.
Answer
\(r=d/t\).
Example\(84\) kilometers in \(2\) hours is \(42\) km/h.
Prompt
What must be true for an out-and-back trip along the same route?
💡Speeds and times may differ.
Answer
The outbound and return distances are equal.
ExampleWrite \(r_1t_1=r_2t_2\).
Prompt
Are outbound and return travel times automatically equal?
Answer
No. Equal distances at different rates require different times.
ExampleThe slower return segment takes longer.
Prompt
Why is \((r_1+r_2)/2\) usually wrong for equal-distance average speed?
💡Average speed is total distance divided by total time.
Answer
The vehicle spends different amounts of time at the two rates.
ExampleEqual distances weight the slower rate for more time.
Prompt
How do you handle a per-hour rate with time in minutes?
💡Divide minutes by \(60\).
Answer
Convert minutes to hours before using \(d=rt\).
Example\(45\) minutes is \(0.75\) hour.
Prompt
What is a useful variable choice for a total-count problem?
💡One variable must control all unknown expressions.
Answer
Choose one unknown count and express every related count in terms of it.
ExampleIf the larger count is \(12\) more, use \(x+12\).
Prompt
What equation structure models a remaining quantity?
💡Keep all terms in the same unit.
Answer
Starting quantity minus amounts used or removed equals the remainder.
Prompt
How should sequential percentage changes be applied?
💡Do not reuse the original amount unless stated.
Answer
Apply each change to the current amount in the stated order.
ExampleAfter a \(20\%\) reduction, the current amount is \(0.8\) times the original.
Prompt
What is a common profit-equation trap?
💡Build complete revenue and cost expressions.
Answer
Subtracting unit prices while ignoring quantities, unsold items, or fixed costs.
ExampleUse \(12(x-5)-7x\), not simply \(12-7\).
Prompt
What does a fraction such as three fifths full describe?
💡Multiply the total by the fraction.
Answer
Three fifths of the unknown total capacity.
ExampleIf \(48\) liters is three fifths full, \(3c/5=48\).
Prompt
How do you model a head-start catch-up problem?
💡One traveler has extra time or distance.
Answer
Set the two traveled distances equal at the catch-up time, including the head start.
Example\(60t=48(t+0.5)\).
Prompt
What check follows a whole-item equation?
💡Do not silently round an incompatible result.
Answer
Verify the solution is a nonnegative whole number and satisfies the original totals.
ExampleA solution of \(14.5\) tickets signals inconsistent data.
Prompt
Why can the solved variable be only an intermediate answer?
💡Re-read the final sentence.
Answer
The question may ask for a related distance, cost, count, or remainder.
ExampleAfter finding time, multiply by rate if distance is requested.
Prompt
What verifies a one-variable word-problem solution?
💡Check the story, not only the simplified equation.
Answer
Substitute it into every original relationship and confirm units and totals.
ExamplePurchased count, sold count, revenue, cost, and profit should all agree.
Prompt
What should be organized separately in a multi-stage travel problem?
💡A table prevents mixing stages.
Answer
Each segment's rate, time, and distance, plus any rest time.
ExampleTotal elapsed time includes travel times and stated stops.
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