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Problem 244

Geometry Difficulty 2.0 Prove it CEMC Galois · Canada · 2024

Three students are helping to expand their school’s garden.
Initially, the garden has a length of 5 m5\text{ m} and a width of 4 m4\text{ m}, as shown.

Rob adds two additional 2 m2\text{ m} by 4 m4\text{ m} plots side by side next to the
initial garden, as shown.

What is the total area of the expanded garden after Rob adds these
two plots?
Kirima adds a path around three sides of
the previous garden, as shown.

If the width of the path is $1\$1\text{}
m}$, what is the total combined area of the garden and the
path?
Noah adds nn additional 2 m2\text{ m} by 4 m4\text{ m} plots to the previous version
of the garden (in part (b)), and then continues the 1 m1\text{ m} wide path so that it surrounds
the entire garden, as shown. If the total combined area of the garden
and the path is 150m2150 \text{m}^2,
determine the value of nn.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

Solution 1:

The length of the expanded garden is (5+2×2) m=9 m(5+2\times2)\text{ m}=9\text{ m}, and the
width is 4 m4 \text{ m}.

Thus, the total area of the expanded garden is $9 m×4 m=36\$9\text{ m}\times4\text{ m}=36\text{}
m}^2$.

Solution 2:

The area of the original $5 \text{}
m}by by 4 \text{} m}$ garden
is $5 m×4 m=20\$5\text{ m}\times4\text{ m}=20\text{}
m}^2$.

Each additional 2 m2 \text{ m} by
4 m4 \text{ m} plot has area 2 m×4 m=8 m22\text{ m}\times4\text{ m}=8\text{ m}^2,
and so the total area of the expanded garden is $(20+2×8) m2=36\$(20+2\times8)\text{ m}^2=36\text{}
m}^2$.
Solution 1:

The combined garden and path has length 9 m+1 m=10 m9\text{ m}+1\text{ m}=10\text{ m}, and
width $(4+2×1) m=6\$(4+2\times1)\text{ m}=6\text{}
m}$.

Thus, the area of the garden and the path is $10 m×6 m=60\$10\text{ m}\times6\text{ m}=60\text{}
m}^2$.

Solution 2:

Consider splitting the path into three rectangles, as shown.

[[IMAGE0]]

Each of the rectangles above and below the garden has dimensions
9 m9\text{ m} by 1 m1\text{ m}, and thus each has area $9 m×1 m=9\$9\text{ m}\times1\text{ m}=9\text{}
m}^2$.

The remaining section of the path has height(4+2×1) m=6 m(4+2\times1)\text{ m}=6\text{ m} and
width 1 m1\text{ m}, and thus has area
$6 m×1 m=6\$6\text{ m}\times1\text{ m}=6\text{}
m}^2. The area of the expanded garden is 36 m^2$, and so the total combined area of the
garden and the path is $(36+2×9+6) m2=60\$(36+2\times9+6)\text{ m}^2=60\text{} m}^2$.
Solution 1:

Each of the new plots has length $2 \text{}
m},andso, and so n$ plots
increase the 9 m9\text{ m} length of
the garden by 2n m2n\text{ m}.

Thus, the combined length of the garden and the path is $(9+2n+2×1) m=(2n+11)\$(9+2n+2\times1)\text{ m}=(2n+11)\text{}
m}. The combined width of the garden and the path is (4+2×1) m=6(4+2\times1)\text{ m}=6\text{} m}$.

Thus in m2\text{m}^2, the total
combined area of the garden and the path is 6×(2n+11)6\times(2n+11).

Solving 6×(2n+11)=1506\times(2n+11)=150, we get
2n+11=1506=252n+11=\frac{150}{6}=25 or 2n=142n=14, and so n=7n=7.

Solution 2:

Consider splitting the combined area of the garden and path into
three rectangles, as shown.

[[IMAGE1]]

Each of the rectangles to the left and right of the garden has height
(4+2×1) m=6 m(4+2\times1)\text{ m}=6\text{ m},
width 1 m1\text{ m}, and thus each has
area $6 m×1 m=6\$6\text{ m}\times1\text{ m}=6\text{}
m}^2$.

The remaining rectangle, which combines the garden and the remaining
sections of the path, also has height 6 m6\text{ m}.

Each of the new plots has length $2 \text{}
m},andso, and so n$ plots
increase the 9 m9\text{ m} length of
the garden by 2n m2n\text{ m}.

Thus, the length of this remaining rectangle is (2n+9) m(2n+9)\text{ m}.

Measured in m2\text{m}^2, the total
combined area of the garden and the path is 2×6+6×(2n+9)2\times6+6\times(2n+9) or 12+6×(2n+9)12+6\times(2n+9).

Solving 12+6×(2n+9)=15012+6\times(2n+9)=150, we
get 6×(2n+9)=1386\times(2n+9)=138 or 2n+9=1386=232n+9=\frac{138}{6}=23 or 2n=142n=14, and so n=7n=7.

Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.