6. Ocean cycles The depth of water, d(t) metres, in a seaport can be approximated by the sine function d(t)= 2.5 sin 0.1641 π(t – 1.5)+ 13.4, where t is the time in hours.

A cruise ship needs a depth of at least 12 m of water to dock safely. For how many hours in each period can the ship dock safely? (to the nearest tenth of an hour)