Why weren't the people on easter island able to grow their own food to survive ?

Because they caused soil erosion, which made the soil inappropriate to plantations.

Explanation:

Large areas were cleared for agricultural land use during the Easter Island population boom, which reached 15,000 inhabitants. By ambition, competition or even lack of knowledge, the natives did not know how to manage their resources in a sustainable way, cleared the entire forest and began to have problems with agriculture.

With the loss of the forest, the sources of water, important for irrigating the crops, have dried up. In addition, the rain and the sun eroded all the soil and this made it impossible for the inhabitants of Easter Island to grow their own food.

50 seconds

t = d /v

where t is the time taken, d is the distance and v is the speed

d= 70 m and s= 1.4 m/s

Therefore;

t = 70/1.4

 = 50 seconds

40 meters

dc = Vc × t

Where dc is the distance down stream, Vc is the velocity downstream and t is the time taken

Vc = 0.8 m/s

t = 50 seconds

Therefore;

dc = 0.8 × 50

    = 40 meters

55 degrees

cosA = 0.8/1.4  

A = 55.15 ° with the horizontal (shore) upstream  

Therefore; the angle to aim from the shore is 55°