With a 225-50-16 tyre, the height of the sidewall is 50% of the width (225mm) of the tyre, so it's 112.5mm
With a 215-65-16 tyre, the height of the sidewall is 65% of the width (215mm) of the tyre, so it's 139.75mm
To retain the same height of the sidewall, when switching from a 215-65-16 tyre to a 225-xx-16 tyre, 'xx' must be 62 (to the nearest whole number).
Chris