轻松计算百分比。找出某数的百分之几，计算百分比增减等。
B% of A = A × B ÷ 100 | A is what % of B? = (A ÷ B) × 100 | Percent change = (B − A) ÷ A × 100 | Percent difference = |A − B| ÷ ((A + B) ÷ 2) × 100 | Percent error = |Observed − Actual| ÷ |Actual| × 100五种百分比模式：求某数的百分比、百分比表示、百分比变化、百分比差异、百分比误差。