Question 166836
lets call the boys rate with the sidewalk=rw   and rw=r+rs r being the rate of the boy and rs being the rate of the sidewalk

lets call the boys rate against the sidewalk and ra=r-rs  

now figuring rates we have rw=303/3=101
:                          ra=275/5= 55

using our equations above we arrive at 101=r+rs
:                                  and  55=r-rs

adding the 1st and 2nd equation together we arrive at 2r=156 so r=78

since r=78 then rs=101-78=23

so the rate of the boy is 78 meters per minute
and the rate of the sidewalk is 23 meters per minute