Find the fourth proportional to 5, 8, and 15.

Problem restatement
For numbers a : b = c : x, find x with a = 5, b = 8, c = 15.

Concept/Approach
Fourth proportional x satisfies 5 : 8 = 15 : x ⇒ x = (8 × 15) ÷ 5.

Step-by-step calculation
x = (8 × 15) ÷ 5 = 120 ÷ 5 = 24

Verification/Alternative
5 : 8 = 15 : 24 → both ratios equal 5÷8 = 0.625 and 15÷24 = 0.625.

Final Answer
24