\(\frac{72+36\times2+24\times3+18\times4+12\times6+168}{2+2+4+6+10+.....+60}\)
\(=\frac{72+72+72+72+72+168}{\left(68-2\right):2+1}\)
\(=\frac{72\times5+168}{\left(68+2\right)\times34\div2}\)
\(=\frac{360+168}{1190}=\frac{528}{1190}=\frac{264}{595}\)
\(b\frac{1991\times1993-1}{1990+1991\times1992}\)
\(=\frac{1991\times\left(1993-1\right)-1}{1990+1991\times1990}\)
\(=\frac{1991\times1992-1}{1990+1991\times1992}\)
\(=\frac{1990}{1990}=1\)