Ans is d) 7th day

In d 1st day Anand does 1/7th of total work

similarly,

Bithu does 1/8th work in d 2nd day

hence at d end of 3 days, work done = 1/7+1/8+1/6=73/168

remaining work = (168-73)/168 = 95/168 

again after 6 days of work, remaining work is = (95-73)/168 = 22/168

and hence Anand completes the work on 7th day.(hope u understood.)

let thet speed of ship is =v

so seaplane speed must be=10v

let seaplane catch the ship in t time

so in t time ship moves x distance

so x/v=(x+18)/10v

x=2

so distance =18+2=20

yarr it's not as difficult as it looks

see

the fastest guy takes 6 days if he is handeling alone ,but it is not hence option for 6 and 3 days is eleminated.

the slowest guy takes 8 days rest all are are fast so there is no way target will go over to 9 days  

hence the only option is 7 days