Henrietta received the bank interest of £6,400. This amount Henrietta received in net so that we need to gross it up to calculate the tax.
The bank is automatically deduct 20% of the amount before it give to Henrietta so that to gross it up we take the amount of £6,400 multiply 100 divide 80.
£6,400 x 100/80=£8,000
According to the scenario, Henrietta received the bank interest is £6,400, because it is a net, so we calculate it to gross. Besides that, the bank is deduct 20% the amount before they give to Henrietta so gross the amount up equals take the amount of bank interest multiply 100 over 80