From the given information, we know that the decreasing function values is

and we need to find the time t when V(t) is equal to $1200. Then by substituting this values into the function, we have

By dividing both sides by 3600, we get

So we have the equations

From the exponents properties, we know that

so we have

or equivalently,

This means that

Then, by dividing both sides by 0.15, we obtain

So, by rounding to the nearest hundreadth, the answer is 6.67 years