The effective rate is calculated in the following way:

where r is the effective annual rate, i the interest rate, and n the number of compounding periods per year (for example, 12 for monthly compounding).
our compounding period is 2 since the bank pays us semiannually(two times per year) and our interest rate is 8%
so lets plug in numbers:
Excel expects a + or = sign before math calculations.
Also, for arguments of functions (such as sqrt), parentheses "()" are used. Brackets "[]" are not acceptable.
That leaves only one remaining option.
50p=c
So $50 x the number of people = the price in dollars
So for 11 people it would cost $550
I hope this helped :)
A=p(1+r/100)^n
A=2500(1+4/400)^16
A=2931.45