This solution was submitted by Sarah from Archbishop Sancroft High School:
The numbers are -1, 1, 3, 5 and 10
-1 + 1 = 0
3 + (-1) = 2
3 + 1 = 4
5 + (-1) = 4
5 + 1 = 6
5 + 3 = 8
10 + (-1) = 9
10 + 1 = 11
10 + 3 = 13
10 + 5 = 15
Solutions were also received from Rachel (West Flegg Middle School), and from Luke and Ian.