a, $\frac{31995 - 81}{42660 -108}$
= $\frac{395 . 81 - 81}{395.108-108}$
=$\frac{81 . 394}{394.108}$
= $\frac{81}{108}$ = $\frac{3}{4}$
b, $\frac{3 . 5 . 7 . 11 . 13 . 37 - 10101}{1212120 + 40404}$
= $\frac{3 . 5 . 7 . 11 . 13 . 37 - 3 . 7 . 13 . 37}{3.7.12.13.37.10 + 3.4.7.13.37}$
= $\frac{(3.7.13.37)(5.11-1)}{(3.7.13.37)(120 + 4)}$
= $\frac{5.11-1}{120 + 4}$
= $\frac{54}{124}$ = $\frac{27}{62}$