In this page combination of solids solution12 we are going to see solution some practice questions of the worksheet combination of solid worksheet2.

**Question 12:**

A hollow cylindrical pipe is of length 40 cm. Its internal and external radii are 4 cm and 12 cm respectively. It is melted and cast into a solid cylinder of length 20 cm. Find the radius of the new solid.

**Solution:**

length of cylindrical pipe (h) = 40 cm

internal radius of the pipe (r) = 4 cm

external radius of the pipe (R) = 12 cm

Height of cylinder = 20 cm

Volume of hollow cylindrical pipe = Volume of cylinder

Π h (R² - r²) = Π r² h

h (R² - r²) = r² h

(40) (12² - 4²) = r² (20)

(40) (144 - 16) = r² (20)

(40) (128)/20 = r²

r² = (40) (128)/20

r² = 2 (128)

r = √256

r = √16 **x** 16

r = 16 cm

**Therefore radius of cylinder = 16 cm**

combination of solids solution12 combination of solids solution12

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