let

b = the average height of mountain B

c = the average height of mountain C

The average height of mountain A is 19,230 ft, and its average height is 8870 ft less tahm the sum of the average heights of mountain B and C:

b + c = 19230 - 8870

b = 10360 - c

The average height of Mountain C is 280 ft less than four-sevenths of the average height of mountain B:

c < (4/7)b

plug b = 10360 - c into the inequality:

c < (4/7)(10360 - c)

by solving we find:

c < 3767.27 ft

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the average height of mountain C is less than 3767.27 ft.