At what height above the ground is g only 64% of its value on the ground?

Step 1. $\left(\dfrac{R}{r}\right)^2 = 0.64$, so $r = \dfrac{R}{0.8} = 8.0 \times 10^{6}\ \text{m}$.

Step 2. Height. $8.0 \times 10^{6}\ \text{m} - 6.4 \times 10^{6}\ \text{m} = 1.6 \times 10^{6}\ \text{m}$, which is $1600\ \text{km}$.