Total output = 7.5×10+8.3×10+102.4 = 260.4 tons.

Average daily output =260.4÷3 1=8.4 tons.

A: The average daily output is 8.4 tons.

Solution:

(350-150) ÷ (1-5/12× 2) =1200 copies.

A: * * * There are 1200 books.

Solution:

External area =π×(20÷2)? = 100π cm2

Internal area =π×( 16÷2)? =64π cm2

Volume = (100π-64π) × 250 = 900π cubic centimeter =28260 cubic centimeter.

Weight = 28260× 7.8 = 220428g = 220.428kg

Answer: The steel pipe weighs 220.428 kg.