Bonjour,
[tex] \frac{500 \times 10^{ - 3} \times 20 \times 10 ^{ - 3} }{ 50\times {10}^{ - 1} \times 200 \times {10}^{2} } \\ \\ \frac{5 \times 10^{2} \times {10}^{ - 3} \times 2 \times 10 ^{1} \times {10}^{ - 3} }{5 \times {10}^{1} \times 2 \times {10}^{2} \times 10 ^{2} } [/tex]
[tex] \frac{10 \times 10 ^{ - 3} }{10 \times 10 ^{5} } = \frac{10 ^{ - 3} }{10 ^{5} } = {10}^{ - 8} [/tex]