6 meters is equivalent to 600 centimeters.
If Willy descends 50cm and back 20 cm
He makes 30 cm per every move
while Sonny climbs 100cm and slips back 50cm
then Sonny makes 50 cm per move.
That will be like this
30 + 50 = 80
600 / 80 = 7.5
So it will take them more than 7 whole day
Willy needs to descend 230 cm(without turning back)and Sonny needs to climb 370 cm(without slipping back) to meet.
To check add both
230 + 370 = 600
To make it clearer,
Willy needs 370 cm descending with 140 cm turning back.
Sonny needs 720 cm of climbing with 350 cm slipping back
I hope it helps