A: You got lucky — the denominator in the saturation efficiency formula must be (T_drybulb – T_wetbulb), which is the maximum cooling possible from an evaporative cooler. Using the dew point instead of the wet bulb gives a larger denominator, which makes the effective delta-T bigger than it should be, and you happened to land on the right answer choice by coincidence. The evaporative cooling process follows a line of constant wet-bulb temperature, so the wet bulb is always the maximum limiting temperature — not the dew point.