Factorise fully 20x^2 - 5

When factorising an expression, you first want to find the highest common factor in each term of the expression. Here, our terms are 20 and 5. The highest common factor here is 5, so we want to take 5 out of the equation first. When we do this, we get 5(4x^2 - 1).
This expression is not yet fully factorised as (4x^2 - 1) can be split into two more brackets because it is a difference of two squares. Both 4 and 1 are square numbers so this can factorise into (2x-1)(2x+1).
Therefore, our final answer is 5(2x-1)(2x+1).