Algebra Question! Please Help!

Question

Estimate (using scientific notation and rounding each entry to one digit) and round off answer to one digit: (5483)(946713×10^-11)/(419723)(0.01502×10^-21)

(A) 6×10¹⁵

(B) 9×10¹⁴

(C) 5×10¹⁶

(D) 7×10^-15

(E) none of these

I had first chosen (E), but got it wrong. Please help me understand this!

Answer 1

5483 is approx equal to 5x10³

946713 is approx equal to 9x10⁵ and so 946713×10^-11 = 9x10^-6 (we add the exponents, 5 + -11)

so (5483)(946713×10^-11) is approx equal to (5)(9)x10³x10^-6 = 45x10^-3

419723 is approx equal to 4x10⁵

0.01502 is approx equal to 2x10^-2 so 0.01502×10^-21 is approx equal to 2x10^-23

so (419723)(0.01502×10^-21) is approx equal to (4)(2)x10⁵x10^-23 = 8x10^-18

Dividing the top and the bottom, 45x10^-3/8x10^-18 we can approximate the number 45/8 as 6 and the exponent becomes -3 - -18 which is 15 so 6x10¹⁵