If a to b is 1 to 2 and b to c is 2 to 3 then a to b to c equals

Let a equal 1k and b equal 2k. Also let b equal 2m and c equal 3m. Equate b to get 2k equals 2m so k equals m. Hence a b c become 1k 2k 3k which reduces to 1 to 2 to 3. Ratios scale by common factors.