The time taken to clean a warehouse varies inversely with the number of cleaners employed.
It takes 8 cleaners 60 hours to clean a warehouse.
Working at the same rate, how many hours would it take 10 cleaners to clean the same warehouse.
- 45
- 48
- 62
- 75
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The time taken to clean a warehouse varies inversely with the number of cleaners employed.
It takes 8 cleaners 60 hours to clean a warehouse.
Working at the same rate, how many hours would it take 10 cleaners to clean the same warehouse.
`B`
`text{Time to clean}\ (T) prop 1/text{Number of cleaners (C)}`
`T=k/C`
`text(When)\ \ T=60, C=8:`
| `60` | `=k/8` |
| `k` | `=480` |
`text{Find}\ \ T\ \ text(when)\ \ C=10:`
`T=480/10=48\ text(hours)`
`=> B`
The number of trees that can be planted along the fence line of a paddock varies inversely with the distance between each tree.
There will be 108 trees if the distance between them is 5 metres.
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a. `90`
b. `4.5\ text(metres)`
a. `t \prop 1/d \ \ =>\ \ t=k/d`
| `108` | `= k/5` |
| `k` | `= 540` |
`text(Find)\ t\ \text(when)\ \ d = 6:`
`t= 540/6= 90`
b. `text(Find)\ d\ text(when)\ \ t = 120:`
| `120` | `= 540/d` |
| `d` | `= 540/120= 4.5\ text(metres)` |
The intensity of light \((I)\), measured in lux, from a lamp varies inversely with the square of the distance from the lamp \((d)\), measured in metres.
At a distance of 2.2 metres from the lamp, the light intensity is 400 lux.
What is the light intensity at a distance of 5.0 metres from the lamp? (2 marks)
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\(77.44\ \text{lux}\)
\(I \propto \dfrac{k}{d^2} \ \Rightarrow \ I=\dfrac{k}{d^2}\)
\(\text{Find k given} \ \ I=400 \ \ \text {when} \ \ d=2.2:\)
\(400=\dfrac{k}{2.2^2} \ \Rightarrow \ k=1936\)
\(\text{Find \(I\) when \(\ d=5\):}\)
\(I=\dfrac{1936}{5.0^2}=77.44\ \text{lux}\)
The quantity \(y\) varies inversely with the square of \(x\).
When \(x = 4, \ y = 10\).
Find the value of \(y\) when \(x = 8\). (2 marks)
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\(y=\dfrac{5}{2}\)
\(y \propto \dfrac{1}{x^2}\ \ \Rightarrow\ \ y= \dfrac{k}{x^2}\)
\(\text{When}\ \ x=4, \ y=10:\)
\(10= \dfrac{k}{4^2}\ \ \Rightarrow\ \ k=160\)
\(\text{Find}\ y\ \text{when}\ x=8:\)
\(y=\dfrac{160}{8^2}=\dfrac{5}{2}\)
A student believes that the time it takes for an ice cube to melt (`M` minutes) varies inversely with the room temperature `(T^@ text{C})`. The student observes that at a room temperature of `15^@text{C}` it takes 12 minutes for an ice cube to melt.
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\begin{array} {|c|c|c|c|}
\hline \ \ T\ \ & \ \ 5\ \ & \ 15\ & \ 30\ \\
\hline M & & & \\
\hline \end{array}
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a. `M=180/T`
b.
\begin{array} {|c|c|c|c|}
\hline \ \ T\ \ & \ \ 5\ \ & \ 15\ & \ 30\ \\
\hline M & 36 & 12 & 6 \\
\hline \end{array}
| a. | `M` | `prop 1/T` |
| `M` | `=k/T` | |
| `12` | `=k/15` | |
| `k` | `=15 xx 12` | |
| `=180` |
`:.M=180/T`
b.
\begin{array} {|c|c|c|c|}
\hline \ \ T\ \ & \ \ 5\ \ & \ 15\ & \ 30\ \\
\hline M & 36 & 12 & 6 \\
\hline \end{array}
The current of an electrical circuit, measured in amps `(A)`, varies inversely with its resistance, measured in ohms `(R)`.
When the resistance of a circuit is 28 ohms, the current is 3 amps.
What is the current when the resistance is 8 ohms? (2 marks)
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`10.5`
`A \prop 1/R\ \ =>\ \ A=k/R`
`text(When)\ \ A=3, \ R=28:`
| `3` | `=k/28` | |
| `k` | `=84` |
`text(Find)\ A\ \text{when}\ R=8:`
`A=84/8=10.5`