A gas mixture contains CH$_4$, C$_2$H$_6$ and H$_2$ with respective concentrations of 75%, 15% and 10% by volume. The lower explosibility limit of CH$_4$, C$_2$H$_6$ and H$_2$ are 5.0%, 3.3% and 4.2% respectively. The lower explosibility limit of the gas mixture, in percentage, is ______.