observed: 10 conversions, 20 non conversions
expected: 30 * 0.03 conversions, 30 * (1 - 0.03) non conversions

Chi = (10 - 30 * 0.03)^2 / (30 * 0.03) + (20 - 30 * (1 - 0.03))^2 / (30 * (1 - 0.03))

Chi = (10 - 30 * x)^2 / (30 * x) + (20 - 30 * (1 - x))^2 / (30 * (1 - x))