If the range of the answer should be +/- 10% of the particular value, then you have to solve for the percentage of that particular value. Since the value given is 1.8, we thus want to find 10% of that:
1.8 * 10/100 = 0.18
Then, add this value to the original value of 1.8:
1.8+0.18 = 1.98
Next, subtract .18 from from the original value of 1.8:
1.8-0.18 = 1.62
The range will be between these two numbers, so the range is from 1.62 to 1.98.
