linear eq.

let

the speed of the boat in still water be x km/h

 and the speed of the stream be y km/h

so speed upstream=(x-y9km/h

speed downstream=(x+y)km/h

speed=dist/time

so

time=dist/speed

so

 time taken to cover 100 km downstraem=100/(x+y)

time taken to go 30 km upstream=30/(x-y)

acc to qn,

100/(x+y)+30/(x-y)=6..(i)

simly

7/(x+y)+7/(x-y)=8..(ii)

take x+y=u

and

x-y=v

solve the eqns for u and v

 then for x and y.

this is the method

but pl chk the second codition in your question once again.Is it 7 km for sure?