If it is said that the range of the answer should be +/- 10% of the particular value, we have to find out the percentage of that particular value. Therefore 10% of 1.8 is,
1.8 * 10/100 = 0.18
If we add this value to the original value, 1.8+0.18 = 1.98
If we subtract this value from the original value, 1.8-0.18 = 1.62