A bakery sells cookies that are made fresh each day. The number of cookies, $C$, sold on a given day can be modeled by the function:
$$C(t) = -2t^2 + 16t + 20$$
where $t$ is the number of days since the beginning of the month. Determine the last day of the month when the number of cookies sold will equal 60. Provide your answer as a decimal number in the grid below.