1,
a, 68.39-168.73+81.68-47.168
=168.(-73-47)+68.(39+81)
=168.(-120)+68.120
=120.(-168+68)
=120.(-100)
b, $\frac{1.5+2.6-3.7+4.8}{2.10+4.12-6.14+8.16}$
$=\frac{5+2^2.3-3.7+2^5}{2^2.5+2^4.3-2^2.3.7+2^7}$
$=\frac{5+2^2.3-3.7+2^5}{2^2.(5+2^2.3-3.7+2^5)}$
$=\frac{1}{4}$