Of the millions who saw the Haley's comet in 1986,how many p

Of the millions who saw the Haley's comet in 1986,how many people will live long enough to see it in twenty-first century
Of the millions who saw Haley’s comet in 1986,how many people will live long enough to see it return in the twenty-first century.
想知道这2句中the的用法,为什么有的加有的不加,都是网上找到的.
还有saw起什么作用呢?