1½ tins is 3/2, so 9/(3/2)=9*2/3=6. So if we reduce the number of tins of yellow by a factor of 6 then we need to reduce the number of red tins by the same amount: 6/6=1 tin of red. (9+6=15 tins=20 litres, so one tin = 20/15 litres=1 1/3 litres.) The ratio of red to yellow is 6:9 or 2:3. So 1 tin of red to one tin of yellow is 1:1½, which is the same as 2:3. (The number of litres is also reduced by 6: 20/6=10/3=3 1/3.)