Resposta :
[tex]\sf 100x=800\cdot \:20\\\\\\100x=16000\\\\\\\dfrac{100x}{100}=\dfrac{16000}{100}\\\\\\x=160\\\\\\R\$800-160R\$=R\$640,00\\\\\\\to \boxed{\sf= \red{R\$640,00} }[/tex]
[tex]\sf 100x=800\cdot \:20\\\\\\100x=16000\\\\\\\dfrac{100x}{100}=\dfrac{16000}{100}\\\\\\x=160\\\\\\R\$800-160R\$=R\$640,00\\\\\\\to \boxed{\sf= \red{R\$640,00} }[/tex]