If i invest R2950, the interest rate 8.10%. What will be my money in period of 18 months?

in principle, it would be either

2950(1+.0810*1.5)
or
2950(1+.0810)^(1.5)

but, if it accrued annually, you will not yet have received the interest for the 6 months of the 2nd year.

As you see, it can get complicated.