We can use 2.5 as compatible with 2.6 and 12 as compatible with 13. 2.5 is the same as 2 and a half so 2.5 times 12 is 24+6=30. We could have used a rougher pair of compatibles: 10 and 3 and multiplied them to get 30. The actual answer is 33.8, so 30 is quite close.