If I understand the problem correctly:

1 Johnny alone could travel 2.5/7 of the trip and return safely, or travel 5/7 th the distance and be stranded,

2 A answer: Let Johnny and 2 friends leave, and at distatance two, each friend gives Johnny provisions for one day, and return with their remaining 2 days provisions. Johny now has 3 day provisions remaining, picks up the 2 day rovisions left by his 2 friends. Johnny has 5 days provisions to travel the remaining 5 day trip.