 there are also some rules to follow if we calculate with numbers that have a limited amount of significant figures. If we multiply or we divide numbers, we would use the smallest amount of significant figures. For example, if I have 4 divided by 2.0, my answer would be 2. Why? Because the number with the lowest amount of significant figures was the 4, it had 1 significant figure. If I had 4.0 divided by 2.0, I would write down 2.0 because 2 significant figures, 2 significant figures, so my lowest amount was the 2, so I write the answer with 2. Now if you add and subtract, we actually do not look at the significant figures but the decimals. For example, 1.3 plus 1.31 would give me 2.6. I would not write the 1 at the end because I round to 1 decimal as this one had only 1 decimal. These rules do not apply for exact numbers. What is an exact number? An exact number is something that is counted or something that is a definition. One foot is 12 inches would be a definition or I have three donuts. If I counted them 1, 2, 3, the 3 would be a counted number. For example, if we have three donuts, each of which has a mass of 25 grams with two significant figures, according to the significant figure rules, if we would take the significant figures of all numbers into account, it should be the lowest one, so 1. I should write the answer with 1 significant figure. However, the 3 in the case of three donuts times 25 grams per donut, the 3 is counted so it doesn't count and my lowest amount of sig figs is in here, which is 2, which gives me 25 grams as an answer. Significant figures to tell you which digit is the uncertain one, which is always the last one, but it will never tell you by how much it is uncertain. Now if you go into the science program, then you will learn an additional method of calculating how much you could be off, so not only will you know where you could be off, but also by how much. Now here the 75, that doesn't tell me if it was 76 or if it could have been 77 or 78 or 72, 71. I don't know, I just know it's the five here and the five here are uncertain. If we would use these uncertainty rules, we would also know plus minus how much, but that's something to worry about if you get to the science program.