This is simple. You thought the kitten weighed 15 pounds but really weighs 8. The first thing you would do is subtract the guess minus the actual. 15 - 8 = 7. That person was off by 7. So you would put it as a fraction over the actual weight. 7/8. Then turn your fraction into a decimal and turn that decimal into a percent. 7 divided by 8 = 0.875. 0.875 as a percent is 87.5%. So the percent error is 87.5%. Hope I helped. :)