Maths Olympiad Prep

Track / Stage 5 / 90 of 400 #690 of 1964

Problem 690

AIME late
Combinatorics Difficulty 5.3 Find the answer

3. From the 10 numbers 0,1,2,3,4,5,6,7,8,90,1,2,3,4,5,6,7,8,9, take out 3 numbers so that their sum is an even number not less than 10. The number of different ways to do this is \qquad.

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

51
3. 【Analysis and Solution】The number of ways to choose 3 different kilometer numbers from these 10 numbers is C53\mathrm{C}_{5}^{3}; the number of ways to choose 1 even number and 2 different odd numbers is C51C52\mathrm{C}_{5}^{1} \mathrm{C}_{5}^{2}.
The number of ways to choose 3 numbers from these 10 numbers such that their sum is less than 10 kilometers is as follows:
(0,1,3);(0,1,5),(0,2,4),(1,2,3),(0,1,7),(0,2,6),(0,3,5),(1,2,5),(1,3,4) (0,1,3) ;(0,1,5),(0,2,4),(1,2,3),(0,1,7),(0,2,6),(0,3,5),(1,2,5),(1,3,4) \text {. }

Therefore, the number of different ways that meet the requirements of the problem is
C53+C51C529=51 (ways).  C_{5}^{3}+C_{5}^{1} C_{5}^{2}-9=51 \text { (ways). }

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.