So to find percent error you want to take the estimated time minus the actual time or, 32 - 40. You get -8. Now take the absolute value of -8, or 8, and divide that by the actual time, 40. 8 ÷ 40 = 0.2. You want to take your decimal, 0.2, and multiply it by 100. 0.2 * 100 = 20. Finally, add the percent sign and you have your percent error: 20%