In a survey of teenagers, ⅔ responded that they got regular exercise, and ⅘ of those said that walking was their main form of exercise. What fraction of the teenagers mainly exercise by walking?

⅔ of the teenagers get regular exercise so ⅔*⅘ = 2/3*4/5 = 8/15 of teenagers mainly walk for exercise. Answer: \boxed{\frac{8}{15}}.

To find the fraction of teenagers who mainly exercise by walking, we need to multiply the fractions representing the two groups of teenagers.

Let's break it down step by step:

1. Start with the fraction of teenagers who responded that they got regular exercise, which is ⅔.

2. Next, we want to find the fraction of teenagers who mainly exercise by walking. According to the information given, ⅘ of those who get regular exercise said that walking was their main form of exercise.

3. Multiply the fractions together: ⅔ * ⅘ = (2/3) * (4/5).

To simplify the multiplication, we can cancel out common factors in the numerator and denominator:

2/3 * 4/5 = (2 * 4) / (3 * 5) = 8/15.

Therefore, the fraction of teenagers who mainly exercise by walking is 8/15.

To find the fraction of teenagers who mainly exercise by walking, we first need to find the fraction of teenagers who get regular exercise.

Given that 2/3 of the teenagers surveyed said they get regular exercise, the fraction can be written as 2/3.

Now, we need to find the fraction of those who get regular exercise and said walking was their main form of exercise.

Given that 4/5 of the teenagers who get regular exercise said walking was their main form of exercise, the fraction can be written as 4/5.

To calculate the fraction of teenagers who mainly exercise by walking, we multiply these two fractions:

2/3 * 4/5 = 8/15.

Therefore, the fraction of teenagers who mainly exercise by walking is 8/15.