According to Amdahl$$s law, what is the percentage of performance improvement if a system administrator increases the number CPUs from $3$ to $6$ for a machine with overall CPU usage at $60\%$

Question $9$Select one:

a$.$

$33\mathrm{.}33\%$

b$.$

$15\mathrm{.}67\%$

c$.$

$10\%$

d$.$

$20\%$