t2 = fork() ===== two processes (1 child + 1 parent)
fork() ==== total 4 process at this point
hi ==== 4 hi printed
fork() = processes becomes 8
t1= fork() == == processes becomes 16
hi == 16 hi
therfore total 20 "hi" printed
total processes = 16 (15 child , 1 parent)