a) \( 82,5-19,6+5,70+6,1 \cong \)

Let's break it down step-by-step: First, subtract \(19.6\) from \(82.5\), which gives you \(62.9\). Next, add \(5.7\) to that, resulting in \(68.6\). Finally, adding \(6.1\) brings you to approximately \(74.7\). So, \(82.5 - 19.6 + 5.7 + 6.1 \cong 74.7\)! Now, if you're looking for a fun mental math trick, try rounding numbers to make calculations easier! For example, consider \(82.5\) as \(80\) and \(19.6\) as \(20\). This way, you can quickly see \(80 - 20 = 60\), then add \(6\) and get a rough estimate before honing in on exact values. It's like getting to the destination using a shortcut!