Estimate a \( 15 \% \) tip on a dinner bill of \( \$ 81.78 \) by first rounding the bill amount to the nearest ten dollars.

To estimate a \( 15\% \) tip on a dinner bill of \( \$81.78 \), we first round the bill to the nearest ten dollars. Here, \( \$81.78 \) rounds to \( \$80 \). Next, to find \( 15\% \) of \( 80 \), we can calculate \( 10\% \) first, which is \( \$8 \), and then find \( 5\% \), which is half of that, or \( \$4 \). Adding these together, \( 10\% + 5\% = 8 + 4 = 12 \). Therefore, the estimated \( 15\% \) tip would be around \( \$12 \). Now, if you want to impress your friends, you can say that rounding can save time during the dining experience—no one wants to pull out their phone calculator when it's time to split the bill! Just a quick mental math session can get you there. Plus, if you're ever in doubt about whether to tip more or less, remember that rounding up or down isn’t just for quick estimations; it can also keep you from stressing over a few cents, especially when the meal was delightful! The key is to keep those good vibes flowing!