let

d = the distance the object travels

t = the length of time it travels

d = k*t, where k is constant

for d = 42 m, t = 18 sec

42 = k*18

k = 42/18

k = 7/3

now we can write

d = (7/3)t

for d = 80 m we need to find t:

80 = (7/3)t

t = 3*80/7

t = 34.29 s

It will take 34.29 seconds to travel 80 meters.