The iPad has become so popular that many people (sorry, Mom!) often say it when referring to any tablet. That reputation is well-earned, given how simple they are to use and how long they typically hold up before needing to be replaced. Thankfully, if you don’t already own one, Apple’s entry-level iPad is on sale at Amazon, Walmart, and Best Buy starting at $279 ($70 off) — an all-time low — for Prime Big Deals Day. If you’ve been sleeping on upgrading an older iPad, or plan to…