The answer to the correct number of significant figures is as follows:
A). (5.3 x 10⁴) + (1.3 x 10⁴)=6.6 x 10⁴
B). (7.2 x 10⁻⁴) /(1.8 x 10³)=4×10⁻⁷
C). 10⁴x 10⁻³ x 10⁶ =10⁷
D). (9.12 x 10⁻¹) - (4.7 x10⁻²)=8.65×10⁻¹
E). (5.4 x 10⁴) x (3.5 x 10⁹)=18.9×10¹³