In a survey of teenagers, 2/3 responded that they got a regular exercise, and 4/5 of those said that walking was their main form of exercise. What fraction of teenagers mainly exercise by walking?
To find the fraction of teenagers that mainly exercise by walking, we need to multiply the fractions of those that exercise regularly and those that mainly exercise by walking.
Given that 2/3 of the teenagers responded that they exercise regularly, and 4/5 of those teenagers mainly exercise by walking, the fraction of teenagers that mainly exercise by walking is:
(2/3) * (4/5) = (2/3) * (4/5)
= (8/15)
Therefore, the fraction of teenagers that mainly exercise by walking is 8/15. Answer: \boxed{\frac{8}{15}}.