31. The blackboard has the following 9 numbers written on it. Each time, the teacher arbitrarily erases two numbers, and writes their sum on the blackboard. When only one number remains on the blackboard, this number is .
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Official solution
answer: 90
Source: NuminaMath-1.5,
licensed Apache-2.0.
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by this site.