Answer: Option 'b' is correct.
Step-by-step explanation:
Since we have given that
Amount of commission = $128
Rate of commission = 5%
Let the amount of his sales be 'x'.
According to question, we get that
[tex]\dfrac{5}{100}\times x=128\\\\0.05x=128\\\\x=\dfrac{128}{0.05}\\\\x=\$2560[/tex]
Hence, Option 'b' is correct.