[tex]b = 5 + f[/tex]
[tex]f = \frac{75}{100} \times b[/tex]
[tex]f = \frac{3}{4} \times b[/tex]
[tex]f = \frac{3b}{4} [/tex]
[tex]b = 5 + \frac{3b}{4} \: | \times 4[/tex]
[tex]4b = 20 + 3b[/tex]
[tex]4b - 3b = 20[/tex]
[tex]b = 20[/tex]
[tex]b = 5 + f[/tex]
[tex]20 = 5 + f[/tex]
[tex]f = 20 - 5[/tex]
[tex]f = 15[/tex]