Answer:
0.51 %
Explanation:
Since mass is the same at sea level and at 13000 m. And weight is the product of mass and gravitational acceleration g, the percent reduction in the weight of an airplane cruising at 13,000 m relative to its weight at sea level is essentially the percent reduction in the gravitational acceleration of an airplane cruising at 13,000 m relative to its gravitational acceleration at sea level, which is:
1 - 9.757 / 9.807 = 1 - 0.995 = 0.0051 = 0.51 %