I need some help with calculating VAT, Net and Total

I read somewhere that when VAT is calculated by taking the figure and dividing it by 6.

AAT has a way of calculating it but I find it confusing. It involves the: figure / 120 x 20. Why is this?

Also, when it comes to calculating the total in the question attached, why is the calculation, " £447 ÷ 100 x 120"?