In a meter bridge 30Ω  resistance is connected in the left gap and a pair of resistance20Ω and Q  in(Ohm) are in right gap and they are connected in series, then the balancing length is 50 cm . When they were connected in parallel ,the balancing length will be

Balancing length l=50 cm Resistance R=30 Ω Balancing condition for meter bridge RS=l100−l 20 Ω  and Q are in right gap (S)  and they are connected in series⇒3020+Q=5050      Q=10Ω 20 Ω  and Q are connected in parallel30(20+Q)20Q=l1100−l1 30×30200=l1100−l1 ⇒92=l1100−l1 900=11l1      l1=90011 ∴  l1=81.81 cm