The given problem is
Let $x$ kilogram of $10 \\%$ chocolate and y kilogram of $30 \\%$ chocolate will be mix
From the given condition we have
$$
x+y=200+\\text { (1) }
$$
Also, $\\quad x \\times \\frac{10}{100}+y \\times \\frac{30}{100}=200 \\times \\frac{13}{100}$
$$
\\text { a.) } \\begin{aligned}
10 x+30 y &=2600 \\
\\text { ar } x+3 y &=260-\\text { (2) }
\\end{aligned}
$$
Swbtracting (2) from (1) we have
$$
3 y-y=260-200
$$
$$
\\text { ar, } \\begin{aligned}
2 y &=60 \\
\\text { ar, } y &=30
\\end{aligned}
$$
from (1) we have
$$
\\begin{aligned}
x &=200-y \\
\\text { ar } x &=200-30 \\
\\text { ar } x &=170
\\end{aligned}
$$
Answer: They mest mix
170 kilograms of the $10 \\%$ chocolate and
30 kilogram of the $30 \\%$ chocolate