A snack stall's board says '2 sandwiches and 3 cups of tea cost Rs 70'. Lower down, the same board says '4 sandwiches and 6 cups of tea cost Rs 150'. What can a customer conclude about the two prices?

Let a sandwich cost Rs $x$ and a cup of tea Rs $y$. The lines say $2x+3y=70$ and $4x+6y=150$. Doubling the first gives $4x+6y=140$, but the board claims 150, so the two lines demand different totals for the very same order. Comparing coefficients, 2/4 = 3/6 = 1/2 while 70/150 = 7/15, so the ratios of the coefficients match but the ratio of the constants does not: the lines are parallel and the pair is inconsistent. Rs 20 and Rs 10 fit the first line only (40 + 30 = 70 but 80 + 60 = 140, not 150). Infinitely many solutions would need the second total to be Rs 140. A third line cannot repair a board that already contradicts itself.