At what rate percent per annum will a sum of R640 amount to R774.40 in 2 years, compounded annually?

To find the rate percent per annum, we can use the formula for compound interest:

A = P(1 + r/n)^(nt)

Where:
A = the amount after t years (R774.40)
P = the principal amount (R640)
r = the annual interest rate (what we are trying to find)
n = the number of times interest is compounded per year (1 for annually)
t = the number of years (2)

Plugging in the values we know:

R774.40 = R640(1 + r/1)^(1*2)
R774.40 = R640(1 + r)^2

Divide both sides by R640:

1.21 = (1 + r)^2

Take the square root of both sides:

√1.21 = 1 + r
1.1 = 1 + r
r = 0.1 or 10%

Therefore, the rate percent per annum is 10%.