Question 1145724
The total length of three cross-country ski trails is 40km.\
y + b + r = 40
 The Yellow trail is 3km longer than twice the length of the Blue trail,
y = 2b + 3
 and the Red trail is 10km shorter than twice the length of the Yellow trail.
r = 2y - 10
 How long is each trail?
:
rearrange the 2nd equation to:
2b = y - 3
b = .5y - 1.5
in the first equation, replace b with (.5y-1.5) and r with (2y-10)
y + (.5y-1.5) + (2y-10) = 40
3.5y - 11.5 = 40
3.5y = 40 + 11.5
3.5y = 51.5
y = 51.5/3.5 
y = 14.7 km is the yellow trail
:
r = 2(14.7) - 10
r = 19.4 km is the red trail
:
b = .5(14.7) - 1.5
b = 5.85 km is the blue trail
:
:
See if they add up
14.7 + 19.4 + 5.85 = 39.95 close enough