In future please double-check your question before posting. Your formula doesn't seem to have come through correctly.
Given that the coupon pays R15 every 6 months for 10 years, we can consider 6 months to be our basic time unit, and so the 6% payable half-yearly is the correct interest rate to use. Effectively we're considering each half-year to be one time unit, thus 6% now applies to one of our time units, the same way an interest rate payable annually applies to one year. Importantly, don't forget to 'convert' the years into our new basic time unit i.e. 10 years in 6 month intervals is 20 basic time units. Thus, we get the formula: