Defective Items in a Production Batch

A factory produces items independently, each defective with probability $p = 0.03$. A batch of $n = 500$ items is inspected. Let $D$ denote the number of defective items in the batch.

Using the Central Limit Theorem, approximate $P(D \le 20)$.

You may use $\Phi(1.30) \approx 0.9032$.