>>>> 0.98 * 7.61 ------> 1 * 8 (not sure about the 0.98 going to 1 on this one)
>>
>>>The fourth example, once again, is correct: rounding to 1 s.d. means, in this case, rounding to the closest 0.1; 0.98 is closer to 1.0 than to 0.9. Or just look at the algorithm in the previos paragraph (the 8 is dropped, 0.9 is increased to 1.0).
>>
>>There is a "fudge" factor inherent in rounding: how big is the difference percentage-wise to the rounded operand.
>>In case of the fourth example I'd be perfectly happy to spare myself the effort to round up to 8 and leave it at 7.61, which is much closer to the actual result. Even true for 1.02 * 7.61, were at least the sides of the cut off parts are eaqulizing and not both skewing in the same direction.
>
>Well, for a 5th. grade course, let's keep it simple... but yes, even the concept of "significant digits" is only a rough approximation. 98 = 100-2, and 102 = 100+2, so the error as a percentage of the total is almost the same (about 2% in each case); but the first has 2 significant digits, the second has 3.

I was not answering to the original question but more on how to get valid ballpark estimates.