If a deposit of P dollars is invested at a rate of interest r compounded continuously for t years, the compound amount is:

A = P * e^(rt)

I = A - P the interest

I = P * e^(rt) - P

I = P(e^(rt) - 1)

e^(rt) - 1 = I/P

e^(rt) = 1 + I/P

rt = ln(1 + I/P)

t = (1/r) ln(1 + I/P)

P = $1100

r = 0.121 or 12.1 annual rate

I = 700

t = the time in years

t = (1/r) ln(1 + I/P)

t = (1/0.121) * ln(1 + 700/1100)

by solving we find:

t = 4.07 years

click here to see the step by step calculation:

in 4.07 years the investment will earn $700 interest.