Let m @ n = m + 3n.

1) What is the value of 13 @ 7?

2) If m @ 26 = 103, what is the value of m?

3) What is the value of 6 @ (6 @ 3)?

4) Simplify the expression:  m @ (m @ n)

5) Simplify the expression: 3t @ {(4s +2) @ (s +t)}