Answer :
You have to cross multiply (multiply the numerator of the first by the denominator of the second and the numerator of the second by the denominator of the first).
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[19]
[tex] \frac{x+4}{3} = \frac{4}{10} [/tex]
[tex]10x+40=12[/tex]
[tex]10x = -28[/tex]
[tex]x=-2.8[/tex]
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[21]
[tex] \frac{4}{n+8} = \frac{10}{8} [/tex]
[tex]32 = 8n + 64[/tex]
[tex]8n = -32[/tex]
[tex]n=-4[/tex]
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[19]
[tex] \frac{x+4}{3} = \frac{4}{10} [/tex]
[tex]10x+40=12[/tex]
[tex]10x = -28[/tex]
[tex]x=-2.8[/tex]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[21]
[tex] \frac{4}{n+8} = \frac{10}{8} [/tex]
[tex]32 = 8n + 64[/tex]
[tex]8n = -32[/tex]
[tex]n=-4[/tex]
[tex]19) \frac{x + 4}{3} = \frac{4}{10} \\ \\ x + 4 = \frac{4}{10} * 3 \\ \\ x = 1.2 - 4 = -2.8 \\ \\ x = -2.8 \\ \\ \\ 20) \frac{4}{n+8} = \frac{10}{8} \\ \\ 32 = 8n + 64 \\ \\ 8n = 32 -64 = -32 \\ \\ 8n = -32 \\ \\ n = -4 [/tex]