1. **State the problem:** Cameron took 4 tests with scores 100, 60, 80, and 30. He took a 5th test scored $x$. The mean of all 5 tests is 72. Find $x$. 2. **Formula for mean:** The mean of $n$ numbers is given by $$\text{mean} = \frac{\text{sum of all scores}}{n}$$ 3. **Apply the formula:** Here, $n=5$ and mean is 72, so $$72 = \frac{100 + 60 + 80 + 30 + x}{5}$$ 4. **Calculate the sum of known scores:** $$100 + 60 + 80 + 30 = 270$$ 5. **Substitute and solve for $x$:** $$72 = \frac{270 + x}{5}$$ Multiply both sides by 5: $$5 \times 72 = 270 + x$$ $$360 = 270 + x$$ 6. **Isolate $x$:** $$x = 360 - 270$$ $$x = 90$$ 7. **Answer:** The value of $x$ is 90, which corresponds to option D.