How can you construct a plane where every point is coloured either black or white such that two points of the same colour are never a unit distance apart?

## Puzzle of the week

## Previous puzzle

Alex and Bob work as financial advisors for the same company. They draw equal salaries from the company. They behave well at the office. Both work on similar assignments. Each assignment requires a yes-no decision. The company uses the decisions made by them to make profits.

After the recession hit the company very badly, one of them has to be fired. Both Alex and Bob have worked on almost the same number of assignments in the last ten years. Alex has been consistently taking about 80% decisions correctly every year. Bob, on the other hand, has been taking only about 5% correct decisions every year.

Assuming that the performances of Alex and Bob would remain the same in future, who should the company fire to maximize its profits in the years to come? Why?

## Random puzzle from the past

There is an \(n\)-dimensional grid in an \(n\)-dimensional Euclidean space made of all points with integer coordinates of the form \((x_1, x_2, \dots, x_n)\) that satisfy the inequality \(0 \leq x_i \leq a_i\) where \(i \in \{1, 2, \dots, n\}\). Every pair of points in the grid that are a unit distance apart are connected by an edge. Through these edges, how many possible shortest paths are there from the point at \((0, 0, \dots, 0)\) to the point at \((a_1, a_2, \dots, a_n\))?