What is the complexity of and how to go about solving the following task:
given $$a,b\in\mathbb{R}_+^n,\ n\ge k\in\mathbb{N}$$ find $$x_{min} \ :=\ \min_{x\in\lbrace0,1\rbrace^n,\ x^Tx=k}\frac{x^Ta}{x^Tb} $$

What is the complexity of and how to go about solving the following task?

Given $a, b \in \mathbb{R}_+^n$ and $n \ge k\in\mathbb{N}$, find

$$ x_{\min} := \arg \min_{x \in \lbrace 0,1 \rbrace^n, x^T x = k} \frac{x^Ta}{x^Tb} $$

What is the complexity of and how to go about solving the following task:
given $$a,b\in\mathbb{R}_+^n$$ find $$x_{min} \ :=\ \min_{x\in\lbrace0,1\rbrace^n,\ x^Tx=k\le n}\frac{x^Ta}{x^Tb} $$

What is the complexity of and how to go about solving the following task:
given $$a,b\in\mathbb{R}_+^n,\ n\ge k\in\mathbb{N}$$ find $$x_{min} \ :=\ \min_{x\in\lbrace0,1\rbrace^n,\ x^Tx=k}\frac{x^Ta}{x^Tb} $$