I'm trying to solve the following equation:

$$12 \times \sin(\frac \pi4t - \frac \pi5) + 20 = 30$$

where $0 \leq t \leq 8$. 

I got this far:

$$ t=\frac{{\sin^-}^1 (\frac 5 6) + (\frac \pi 5) + n \times 2\pi}{\frac \pi 4} $$

I do get $t\approx 2,1$, but the solutions say I should also get $t\approx 3,5$. 

I cannot for the life of me figure out how?