50 meters
Explanation
you can easily solve this by using a rule of three.
Step 1
Let
[tex]\begin{gathered} 1\text{ milemeter}\rightarrow5\text{ meters} \\ \text{the proportion is} \\ \frac{1\text{ mm}}{5\text{ mt}} \end{gathered}[/tex]x represents the length of the actual lawn
then
[tex]\begin{gathered} 10\text{ mm}\rightarrow x\text{ mt} \\ \text{the proportion is} \\ \frac{10\text{ mm}}{\text{xmt}} \end{gathered}[/tex]the proportion is the same, so
Step 2
[tex]\begin{gathered} \frac{1}{5}=\frac{10}{x} \\ \text{solve for x} \\ 1\cdot x=5\cdot10 \\ x=50 \end{gathered}[/tex]so, the length is 50 meters