In anything. Anywhere you see a percentage you can convert it to a fraction. Approximately 1/5th of our atmosphere is oxygen gas. 2/3s of the species on Earth can be found in tropical regions.

Speed is distance over time. Density is mass over volume. You can scale things up and down using ratios. Converting units requires conversion factor ratios. Fractions are probably one of the most used pieces of math in science.

Physical Science: 1/Req = 1/R1 + 1/R2 + 1/R3 + ..... + 1/Rn, where Req is the equivalent resistance of a parallel circuit made up of resistors, R1, R2, R3, ...., Rn. Then with the diagram and equation it is easy to show why adding a resistor decreases the "total" resistance. 1/f = 1/do + 1/di where f is the focal length of the mirror or lens, do is the distance to the object, di is the distance to the image. Solve for f: 1/f = di/(do)(di) + do/(do)(di) to get common denominators; 1/f = (di + do) / (do di) adding fractions; f = (do di) / (di + do). Find the % composition of various compounds such as HCl, NaOH, C3 H8 or generally CxHy. For example, take HxSOy where x is 2 and y is 4. Last I checked, H is 1.00797, S is 32.06400 and O is 15.99940. H: 1.00797 x 2 = 2.01594 S: 32.06400 x 1 = 32.06400 O: 15.99940 x 4 = 63.99760 Molecular Mass =98.07754. % H = 2.01594/98.07756 * 100% = about 2.05546%, %S = 32.06400/98.07756 * 100% = about 32.69250%, %O = 63.99760/98.07756 * 100% = about 65.25204%. Also there are plenty of easy %W/V, W/W calculations/ The swinging pendulum will divide that wall clock time into ten, twenty, hundred or any number of parts. Of course you will want to have groups do many different times so as to show that the period of the pendulum stays very consistent (close to the same) even though the displacement decreases from swing 1 to 100. The acceleration timer: How many seconds for each dot? For example, suppose the following dots were made in 10 seconds: . . ... . . . . . . .......... .. .. ................................... .................... .......... ........................ ................ .......... Tear the timer tape into 2 - 7 parts depending on how many groups you have, have each group count and add up the dots. I think there's 140. If 140, then the period of the timer is 10s/140 or 1/14 of a second. Then use the timer and fraction to time other events. Ha-int. This has been fun. (LOL)