The average (arithmetic mean) of 5, 10, 15 and x is 20. What is the value of x?

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The average (arithmetic mean) of 5, 10, 15 and x is 20. What is the value of x? Answer: 50

That makes it 4 x 20 to get the total sum which is 80 80=5+10+15+x 80–30=x 50=x