Approximately 10 hours
1) Gathering the data
Initial temperature: 79º F
0.8 each hour
84º
2) We can write an exponential function to find that out. So let's do it this way:
[tex]\begin{gathered} T_f=T_0(1+0.8)^h \\ 84=79(1.8)^h \\ \frac{84}{79}=\frac{79(1.8)^h}{79} \\ 1.06=1.8^h \\ \log _{10}1.06=\log _{10}1.8^h \\ h\log _{10}1.8=\log _{10}1.06 \\ h=10.087 \end{gathered}[/tex]Notice that since the temperature is rising, we have to add one to the factor, otherwise, it will decrease it.
Now let's convert that decimal number so that we may have a better approximation:
10 hours and 4 minutes