If the sum of 5, 8, 12, and 15 is equal to the sum of 3, 4, x, and x + 3, what is the value of x?

Question: If the sum of 5, 8, 12, and 15 is equal to the sum of 3, 4, x, and x + 3, what is the value of x?

Solution:
5+8+12+15 = 40

Now,
40 = 3 + 4 + x + x + 3
⇒ 40 = 10 + 2x
⇒ 30 = 2x
⇒ 30/2 = x
∴ 15 = x