Thanks Michael,
Your first bit is good but then I am not sure what you did ://
Anyway, I want to take a quick look myself. :)
\( 60.1 − 43.9 cos\frac{ πt}{ 6}\ge 77\\ − 43.9 cos\frac{ πt}{ 6}\ge 16.9\\ cos\frac{ πt}{ 6}\le -16.9/43.9\\ cos\frac{ πt}{ 6}\le -0.385\\ \frac{ πt}{ 6}\le cos^{-1}(-0.385)\\ \frac{ πt}{ 6}\ge 1.966\\ πt\ge 11.796\\ t\ge3.75 \)
I will assume that 1 is the END of January SO 4 is the end of April.
So sales will be exceeded in April
Here is a graph that might help you to understand. Although the equation itself actually makes little sense.
