$(\frac{3}{4}a^3b) \; (\frac{2}{5}abc)$=
=$\frac{3\cdot 2}{4\cdot 5} a^{(1+3)}b^{(1+1)}c$ =
=$\frac{6}{20}a^4b^2c=\frac{3}{10}a^4b^2c$ .
----------------------------------------------------------------------------------
$\big(\frac{3}{4}a^3b\big)\big(\frac{2}{5}abc\big)$ =
= $\frac{6}{20}a^4b^2c$ =
= $\frac{3}{10}a^4b^2c$
+ 6/20 a^4 b^2 c = 3/10 a^4 b^2 c
3*a^4*b^2*c / 10
+3/10 a^4b^2c