Answer: 1.29 x 104 lbs.
Explanation:
4.23 x 103 lbs. and 8.7 x 103 lbs.
4.23 x 103 + 8.7 x 103 =
4.23 x 10^3 + 8.7 x 10^3 =
(4.23+8.7) x 10^3 =
(4.23+8.70) x 10^3 =
(12.93) x 10^3 =
1.293 x 10 x 10^3 =
1.293 x 10^1 x 10^3 =
1.293 x 10^(1+3) =
1.293 x 10^4 =
1.293 x 10^4 = 1.29 x 104 lbs.