Question

qual deve ser o capital aplicado a uma taxa de juros simples de 25% a.a.para que em 6 meses renda R$425,00 de juro?

Answer 1

j = 425
i = 25% a a =  25/100 = 0,25
t = 6 m  = 6/12 do ano  = 1/2 do ano  ou  0,5 do ano  
j = cit
425 = C * 0,25 * 0,5
425 = 0,125C
C = 425/0,125 = 3 400 ****

Answer 2

$\sf\large \boxed{\boxed{\tt J = C \cdot i \cdot t}}$

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$\sf \displaystyle 425\:=\:C\:\cdot 0,\:25\:\cdot \left(\frac{1}{2}\right)\\\\\\\sf \boxed{\star \sf \ {Troque \:os \:lados} }\\\\\\\sf C\cdot \:0.25\left(\frac{1}{2}\right)=425\\\\\\\sf C\frac{0.25}{2}=425\\\\\\\sf 2C\frac{0.25}{2}=425\cdot \:2\\\\\\\sf 0.25C=850\\\\\\\sf \frac{0.25C}{0.25}=\frac{850}{0.25}\\\\\\\\\large \to \boxeed{\boxed{\sf C=3400}}$