15. 12 per cent. Prime cost 5s. 6d. per lb.

4. 10x+9a-7c-6d-5f.

(a1) 23-(bp) x2 + (c + q) x − d + r.

2n

10. a3m+3a2m x" +3 am x2n + x3n.

11. A. (a+b+c). (a+b−c) (a+bc). (a−b+c).

B. (x + y) · (x − y) (x*+y*). (x* −y3) (x*+y†). (2* —y‡)

x2. (2a — 3x). (3 a + x).

C. xy. (x+2y). (2x+y).

6.

5. A. x = 2 or ().

${(4a2

A. x = 5, y −3. x = † {(4 a2 + b2)$ + b}, y = } {(4 a2 + b2)* — b}.

—

B. x = 5, y

= 3. x = 3, y = 2.

3. A. a2 — 6 a3 b3 + 21 a3 b3 — 44 a b‡ + 63 aa3 ¿a3 — 54 a3 b3 + 27 b.