Perform the following calculations and express the answers in
scientific notation.
a. 4.2 x 104 kg + 7.9 x 10³ kg
b. (5.23 x 106 um) (7.1 x 10-² μm)
c. 5.44 x 107 g + 8.1 x 104 mol
d. 8.40 x 105 km - 3.1 x 105 km