arithmetic

let total water W = wn+ws  (wn/ws = water in northern/southern hemisphere)

and total earth E = en+es  (en/es= land in northern/southern hemisphere)

then E/W = 1/2   ==>E=W/2  or W=2E

and en/wn = 2/3 ==> en= 2/3wn  or wn = 3/2en

let es/ws = k ==> es = k ws  or ws = es/k

now E = en+es 

W/2 = (wn+ws)/2 = 2/3wn + k ws

-wn/6 = (k-1/2)ws.....(1)

and W = wn+ws

2E = 2(en+es) = 3/2en + es/k

en/2 = (1-2k)es/k.....(2)

divide (1) by (2)

we know en/wn and es/ws = k 

the only variabel will be k. solve for it.