To solve for \( m \) in the equation \( a = \frac{F}{m} \), we need to isolate \( m \) on one side of the equation. This can be done by multiplying both sides of the equation by \( m \) and then dividing both sides by \( a \).
Step 1: Given Equation
We start with the equation relating acceleration \( a \), force \( F \), and mass \( m \):
\[
a = \frac{F}{m}
\]
Step 2: Rearranging the Equation
To isolate \( m \), we multiply both sides by \( m \):
\[
a \cdot m = F
\]
Next, we divide both sides by \( a \):
\[
m = \frac{F}{a}
\]
Step 3: Substituting Values
We substitute the given values \( a = 5 \) and \( F = 20 \) into the equation:
\[
m = \frac{20}{5}
\]
Step 4: Calculating the Result
Calculating the right-hand side gives:
\[
m = 4.0
\]
Final Answer
Thus, the value of \( m \) is
\[
\boxed{m = 4.0}
\]