Explanation:

- Let's consider the 5 consecutive even numbers as \(x, x+2, x+4, x+6, x+8\).

- The last 3 numbers are \(x+4, x+6, x+8\).

- The average of these is \(\frac{(x+4) + (x+6) + (x+8)}{3} = x+6\).

- The square of the average is \((x+6)^2\).

- We're given \((x+6)^2 = x^2 + 420\).

- Solving gives \(x^2 + 12x + 36 = x^2 + 420\).

- Simplifying gives \(12x + 36 = 420\).

- Solving gives \(12x = 384\), which means \(x = 32\).

- The smallest number is 32, and the largest is 40.

- Their sum is \(32 + 40 = 72\).

Option:4, 72