\fontsize{15}{18}\selectfont
Problem Statement
Solution:
\documentclass[a4paper,12pt]{article}
\usepackage[left=2cm, top=2cm, right=2cm, bottom=1cm, includeheadfoot,
headheight=50pt]{geometry}
\usepackage{fancyhdr}
\usepackage{lastpage}
\usepackage{float}
\usepackage[most]{tcolorbox}
\usepackage{enumitem}
\usepackage{graphicx}
\usepackage{amsmath, amsthm}
\usepackage{fontspec}
\usepackage{unicode-math}
\setmainfont{Linux Libertine O}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}{Lemma}[section]
\newcommand{\Name}{Hostek}
\newcommand{\Email}{your.email@example.com}
\newcommand{\ProblemNumber}{XYZ OM, etap A, zadanie B}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[L]{\Name \\ \Email}
\fancyhead[C]{\ProblemNumber}
\fancyfoot[C]{\thepage/\pageref{LastPage}}
\renewcommand{\headrulewidth}{0.4pt}
\renewcommand{\footrulewidth}{0.4pt}
\begin{document}
\fontsize{15}{18}\selectfont
\section*{Problem Statement}
\bigskip
\noindent\textbf{Solution:}
\end{document}