If the prices of sawtimber increased by 33.33%, in which year was the difference in prices of one tonne of saw timber and one cubic metre of logs the least?

After an increase by 33%, the difference in the prices of one tonne of saw timber and one cubic metre of logs is given below according to the respective years.

The difference in 1987 is 15 - 10*133.33% = 1.67
The difference in 1988 is 16 - 10*133.33% = 2.67
The difference in 1989 is 17.5 - 12*133.33% = 1.5
The difference in 1990 is 15 - 10*133.33% = 1.67
The difference in 1991 is 17.5 - 12.5*133.33% = 0.833
The difference in 1992 is 18 - 15*133.33% = -2
The difference in 1993 is 20 - 20*133.33% = -6.67

Hence, the difference is least in the year 1991.