Ataxi travels 25 kilometers east of an airport then it travels from that point to a point that is 40 kilometers west of the airport finally the taxi returns to the airport how far did the taxi travel

(1)    25 kilometers  --> due east
(2) To travel 40 kilometers west of the airport, it still has to cover the return trip from distance (1). Summing up will give us an answer of 65 kilometers. 
(3) To return to the airport, it has to cover the 40 kilometers from west of airport, 40 kilometers.

Summing them all up will give us an answer of 130 kilometers.

Because both x and y are expressed to the FIRST power, this relationship is linear.  It is not proportional, however, since x + y = 20 does not have the format y = kx (where k is the constant of proportionality).