Starts at 20 so look for y-intercept of 20

Leaves answers (2), (3) and (4)

Doubles every 10 years so must go up.

Leaves answers (3) and (4)

Which one looks like doubling every 10 years

(0, 20) is the starting point

(10,40) ten years and it doubles

(20,80) ten more years and it doubles again

(30, 160) ten more years and it doubles again

(40, 320) ten more years and it doubles again

Now we are off the graph!!