Final answer:
To model the variation described, we use the formula a = k * (b / c). By substituting the given values and solving for k, we find k = 1/8. Using this value of k, we can find a when b = 18 and c = 9 to be 1/4.
Step-by-step explanation:
To model the variation described, we can use the formula a = k * (b / c), where k is the constant of variation. Given that a varies directly with b and inversely with c, we can write the equation as a = k * (b / c).
Using the given information, we can substitute the values of b = 10, c = 20, and a = 1/4 into the equation and solve for k. This gives us k = (a * c) / b = (1/4 * 20) / 10 = 1/8.
Finally, we can use this value of k to find a when b = 18 and c = 9 by plugging them into the equation: a = (1/8) * (18 / 9) = 1/4.
Learn more about Direct and Inverse Variation