[tex] F1 = \frac{kq}{ {r1}^{2} } = > q = \frac{ {r1}^{2} \times F 1 }{k} = \frac{ {0.5}^{2} \times 16}{9 \times {10}^{9} } = 0.4 \times {10}^{ - 9} C[/tex]
[tex]F2 = \frac{kq}{ {r2}^{2} } = \frac{9 \times {10}^{9} \times 0.44 \times {10}^{ - 9} }{ {4}^{2} } = \frac{4}{16} = 0.25 \frac{N}{C} [/tex]