3. To surf the internet for 15 minutes at an airport, it costs $\mathrm{\$}4.05$. For 40 minutes, it costs $\mathrm{\$}5.80$. Write and solve a linear equation to find the cost for surfing the web for one hour.
14. Water boils at ${100}^{\circ }$ Celsius or ${212}^{\circ }$ Fahrenheit. Water freezes at $0^{\circ }$ Celsius or ${32}^{\circ }$ Fahrenheit. If the weather forecaster says it will be ${25}^{\circ }$ Celsius today, write and solve a linear equation to find what Fahrenheit temperature this is.

Step 1 ：Let's denote the cost for surfing the internet for $x$ minutes as $C(x)$.

Step 2 ：We know that $C(15)=\mathrm{\$}4.05$ and $C(40)=\mathrm{\$}5.80$.

Step 3 ：We can write the equation of the line passing through the points $(15,4.05)$ and $(40,5.80)$.

Step 4 ：The slope of the line is $\frac{5.80-4.05}{40-15}=\frac{1.75}{25}=\mathrm{\$}0.07$ per minute.

Step 5 ：So, the equation of the line is $C(x)=0.07x+b$.

Step 6 ：To find $b$, we can substitute one of the points into the equation. Let's use $(15,4.05)$:

Step 7 ：$4.05=0.07\cdot 15+b$,

Step 8 ：$b=4.05-0.07\cdot 15=\mathrm{\$}2.95$.

Step 9 ：So, the cost for surfing the internet for $x$ minutes is $C(x)=0.07x+2.95$.

Step 10 ：To find the cost for surfing the web for one hour (or 60 minutes), we substitute $x=60$ into the equation:

Step 11 ：$C(60)=0.07\cdot 60+2.95$

Step 12 ：$\boxed{{\displaystyle C(60)=\mathrm{\$}7.15}}$

Step 13 ：We know that $f=32+1.8c$.

Step 14 ：To find the Fahrenheit temperature when it is ${25}^{\circ }$ Celsius, we substitute $c=25$ into the equation:

Step 15 ：$f=32+1.8c$

Step 16 ：$f=32+1.8(25)$

Step 17 ：$\boxed{{\displaystyle f={77}^{\circ }}}$