Maths problem: how to do, thanks?

a) 令 C = x + k/n,則
73000 = x + k/100 ....(1)
43000 = x + k/200 ....(2)
(1)-(2)得30000 = k/100 - k/200
6000000 = k,代入(1)得
73000 = x + 60000
x = 13000
故 C = 13000 + 6000000/n

b) $(13000 + 6000000/400)/400
= $70

c) 設p為所求人數,則
(13000 + 6000000/p)/p = 50
(260 + 120000/p)/p = 1
p² - 260p - 120000 = 0
p² - 2(130)p + 130²= 120000+130²
(p - 130)² = 136900
p - 130 = 370 或 -370(捨去)
p = 500
答:500人