Maths problem: how to do, thanks.?

依題意 C = ar + br^2
依所給條件, 得兩方程式:(1) 202 = 10a+100b(2) 51 = 5a + 25b(1) - (2)×2 得   100 = 50b∴ b = 2∴ a = (202-100×2)/10 = 0.2(a)r = 15 則 C = 15×0.2 + 225×2 = 453The cost is $453.(b)Surface area varis as r^2. (area = 4πr^2)這表示此圓球飾物半徑為 (a) 次兩倍, 即 30cm.∴ C = 30×0.2 + 900×2 = 18061806 ≒ 453×4 但 1806 ≠ 453×4∴ Betty 的宣稱近似是對的, 但不完全正確.   也就是說, 表面積變為4倍, 生產成本也大約   變為4倍, 但不是剛好4倍.