15.8+15.4/12.3 = 17.0520325203
\(\frac{15.8+15.4}{12.3}=\frac{15.\left(8+4\right)}{12.3}=\frac{15.12}{12.3}=5\)
\(\frac{15\cdot8+15\cdot4}{12\cdot3}\)
\(=\frac{15\cdot\left(8+4\right)}{12\cdot3}\)
\(=\frac{15\cdot12}{12\cdot3}\)
\(=\frac{15}{3}\)
\(=5\)