Solution: We know that resultant molarity is given by
Resultant Molarity = (M1V1+M2V2)/(V1+V2)
we have M1 = 0.1, V1 = 40ml = 0.04 litre
M2 = 0.2, V2 = ?
Now putting the values in above formula we get,
(0.1x0.04 + 0.2xV2)/0.04+V2 = 6/55
=> 0.004+0.2V2 = (0.24+6V2)/55
=> 0.22+11V2 = 0.24+6V2
=> 5V2 = 0.02
=> V2 = 0.004 litre
Hence, V2 = 0.004 litre or 4ml