b
Initially

Charge on capacitor 10 \(\mu F\)

\(Q=C V=(10 \mu F)(50 V)\)

\(Q =500 \mu C\)

Final Charge on \(10 \mu F\) capacitor \(Q = CV =(10 \mu F )(20 V )\)

\(Q =200 \mu C\)

From charge conservation,

Charge on unknown capacitor

\(C =500 \mu C -200 \mu C =300 \mu C\)

\(\Rightarrow\) Capacitance \(( C )=\frac{ Q }{ V }=\frac{300 \mu C }{20 V }=15 \mu F\)