Estimate the values of fvv(30, 20), ftt(30, 20), fvt(30, 20), and ftv(30, 20). (WARNING: These calculations will require some thought.)

4. The wave heights h in the open sea depend on the speed v of the wind and the length of time t that the wind has been blowing at that speed. Values of the function h = f(v, t) are recorded in feet in the following table. ,, 5 10 Duration (hours) 15 20 2 2 30 2 Wind speed (knots) 9 13 16 17 18 1919 40 14 21 25 28 29 36 45 48 24 37 47 54 62 67