\(4(2x-5)\) is equal to \(8x-20\).
Because, when multiplying an integer with a bracket, you multiply the integer with all the stuff in it.
Like this,
\(a(bx+c)=\quad a\cdot bx+b\cdot c\)
So if we expand the expression \(4(2x-5)\)
\(4(2x-5)=\quad 4\cdot 2x+(4\cdot -5)\quad \Longrightarrow \quad 8x+(-20)\quad \Longrightarrow \quad 8x-20\)