From outside inward, Earth has crust, mantle, outer core and inner core. The outer core is liquid; the inner core is solid. Plates are pieces of lithosphere: crust plus rigid uppermost mantle. One plate can carry oceanic crust, continental crust or both, so a plate is not simply one continent.
Most mantle is solid but can deform slowly over geological time. Plates move relative to one another, often by centimetres per year. At a stated constant rate, distance equals rate multiplied by time. Rate equals distance divided by time. Small annual displacements accumulate over long periods; real rates and directions need not remain constant forever.
Example 1
A plate carries a continent and nearby ocean floor. Is that possible?
Yes. One lithospheric plate can contain both continental and oceanic crust.
Example 2
A plate moves 3cm each year for 10 years at a constant rate. Find its displacement.
3×10=30cm in the stated direction.
Watch out: Plates are lithosphere and may include ocean floor. Most mantle is solid but can deform very slowly.