Why: The downhill force that pushes on each toboggan is caused by gravity and is thus proportional to the mass of each toboggan.
(The downhill force; pushes; on each toboggan) (The downhill force that pushes on each toboggan; is; thus proportional to the mass of each toboggan)
The downhill force The downhill force that pushes on each toboggan
pushes is
on each toboggan
thus proportional to the mass of each toboggan