The Seasonal Index Nobody Reported

A retailer models quarterly revenue (in thousands of dollars) with the classical additive decomposition

$y(t) = T(t) + S(t) + R(t)$

The trend has been estimated as the straight line

$T(t) = 200 + 6t$

where $t$ is the quarter index, with $t = 1$ meaning Q1 of year 1, $t = 2$ meaning Q2 of year 1, and so on through consecutive quarters.

Three of the four quarterly seasonal components were reported:

$S_{Q1} = -15, \qquad S_{Q2} = +25, \qquad S_{Q3} = +30$

The value of $S_{Q4}$ was left out of the report. Using the standard convention that the classical additive model imposes on seasonal components, forecast the revenue at $t = 16$, taking the residual to be $0$.