Problem Solving

Each of the integers from 0 to 9, inclusive, is written on a separate slip of blank paper and the ten slips are dropped into a hat. If the slips are then
drawn one at a time without replacement, how many must be drawn to ensure that the numbers on two of the slips drawn will have a sum of 10?

(A) Three
(B) Four
(C) Five
(D) Six
(E) Seven

IMO 7

to add 10 we have to have 9,1; 8,2; 7,3; 6,4
in the worst situation we'd get 0,1,2,3,4,5,6 (7 slips) to add exactly 10...
HTH