I had a quick search on the forums for help on my question and found this http://www.mathhelpforum.com/math-he...ays-tides.html

However my question is a little different in terms of the placement of "t" and the fact that i'm useless with fractions in equations.

A large ship can only enter the harbour when the depth of water is at
least 1.3 metres above the horizontal mark. At what times during the
week commencing 30 March can the ship enter the harbour?
The equation is $\displaystyle 2.5 \sin \left(\frac{\pi t}{12}\right)$

The question in the link is similar but with the placement of the "t" in the equation. I know from my graph and workings that 1.250m occurs at Hours 2, 10, 26 and 34 for 0 ≤ t ≤ 36.

I tried to rearrange the equation for 1.3 by doing:
2.5sin(pi)t/12 = 1.3
sin(pi)t/12 = 0.52

but then i got stuck. Any inputs or hints pls?