Answer: 10%
Step-by-step explanation:
The City of Helfin bond is a municipal bond so it is tax exempt. The Surething bond is not however so the interest rate that would make Hugh indifferent is one that would equate the after tax return on the Surething bond to that of the City of Helfin.
6% = x ( 1 - Tax)
6% = x ( 1 - 0.4)
x = 6%/0.6
= 10%