[tex]70km[/tex]
1) Whenever we have problems that deal with the real distance and distance on the map, we can write out the following formula:
[tex]\begin{gathered} Scale=\frac{distance\: on\: the \: map}{real\: distance} \\ S=\frac{3.5}{R} \end{gathered}[/tex]
So, let's call the Real distance "R". Note that in this question, 1 centimeter represents 20 kilometers
2) So, in order to plug that as the scale we need to convert kilometers to centimeters:
[tex]\begin{gathered} 1km--100000cm \\ 20km---x \\ x=2000000cm \end{gathered}[/tex]
3) Finally, we can plug that into the formula and cross multiply the ratios this way:
[tex]\begin{gathered} \frac{3.5}{r}=\frac{1}{2000000} \\ r=2000000\times3.5 \\ r=7000000cm \end{gathered}[/tex]
Converting it back to kilometers we can write:
[tex]\frac{7000000}{100000}=70km[/tex]
