The question states assume no change to G spread, and then calculates a new interpolated rate for the government yield based on the 0.2% increase.
Wouldn’t this change the G spread compared to what it was without the 0.2% increase?
But they have told you it does not so you can assue the chnage in the YTM of the corporate bind is only due to the interpolated ytm of a gvt 12 year bond.
if the g-spread stays the same then the YTM of the corporate bond will increase by 4bp. (0.2 x 20bp)