see. in wurtz reaction .. one mol of butane is formed from 2 noles of ethane bromide ..
now. 55 gm butane i.e. 55/58 moles of butane are formed .. so no of moles of ethane bromide rqd = 2*55/58
now .. efficiency of wurtz is 85% . so actual moles of ethane bromide reqd is (100/85)*(2*55/58)
efficiency of bromination is 90% hence acual moles of ethane reqd is (100/90)*[(100/85)*(2*55/58)] = a (say)
so. moles of ethane reqd at ntp = 22.4*a litres