This can be written as C ∝ n. The constant of proportionality is 20, so C = 20n. For example, if n = 3, C = 20 × 3 If C = 20 × 3, the cost is 60p. Two variables are directly proportional when ...

For example, a caterpillar may exert a direct effect on a plant by eating it. If a bird eats that caterpillar (also a direct effect), this can reduce damage to the plant, resulting in an indirect ...