[tex]a + b = 28[/tex]
[tex]a = \frac{2}{5} \times b[/tex]
[tex]a = \frac{2b}{5} [/tex]
[tex] \frac{2b}{5} + b = 28 \: | \times 5[/tex]
[tex]2b + 5b = 140[/tex]
[tex]7b = 140 [/tex]
[tex]b = \frac{140}{7} [/tex]
[tex]b = 20[/tex]
[tex]a = \frac{2b}{5} [/tex]
[tex]a = \frac{2 \times 20}{5} [/tex]
[tex]a = 2 \times 4[/tex]
[tex]a = 8[/tex]