Lindsay finds the perimeter of a figure when a side length is not given. Practice this yourself on Khan Academy right now: https://www.khanacademy.org/math/cc-third-grade-math/cc-third-grade-measurement/cc-third-grade-perimeter/e/find-a-missing-side-length-when-given-perimeter Watch the next lesson: https://www.khanacademy.org/math/cc-third-grade-math/cc-third-grade-measurement/cc-third-grade-compare-area-perimeter/v/comparing-area-and-perimeter Missed the previous lesson? https://www.khanacademy.org/math/cc-third-grade-math/cc-third-grade-measurement/cc-third-grade-perimeter/v/perimeter-of-a-shape 3rd grade math on Khan Academy: We know you've been rocking through 2nd grade adding and subtracting all kinds of whole numbers (up to 2 digits, right?). That's awesome! In 3rd grade math we want you to start using bigger numbers and start multiplying and dividing, too. By the way, did you know that some numbers aren’t actually “whole?” They’re “partially whole.” We call them fractions! We want you to start playing around and having fun with those, too. There's also area, perimeter, and place value to be discovered. Whew. We have so much to do and can't wait to do it with you. Let's go! ... https://www.youtube.com/watch?v=tmt5fXhU7Rs Note: All Khan Academy content is available for free at (www.khanacademy.org)

Watch the next lesson: https://www.khanacademy.org/math/pre-algebra/applying-math-reasoning-topic/alternate-number-bases/v/decimal-to-hexadecimal?utm_source=YT&utm_medium=Desc&utm_campaign=PreAlgebra Missed the previous lesson? https://www.khanacademy.org/math/pre-algebra/applying-math-reasoning-topic/alternate-number-bases/v/decimal-to-binary?utm_source=YT&utm_medium=Desc&utm_campaign=PreAlgebra Pre-Algebra on Khan Academy: No way, this isn't your run of the mill arithmetic. This is Pre-algebra. You're about to play with the professionals. Think of pre-algebra as a runway. You're the airplane and algebra is your sunny vacation destination. Without the runway you're not going anywhere. Seriously, the foundation for all higher mathematics is laid with many of the concepts that we will introduce to you here: negative numbers, absolute value, factors, multiples, decimals, and fractions to name a few. So buckle up and move your seat into the upright position. We're about to take off! About Khan Academy: Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. We tackle math, science, computer programming, history, art history, economics, and more. Our math missions guide learners from kindergarten to calculus using state-of-the-art, adaptive technology that identifies strengths and learning gaps. We've also partnered with institutions like NASA, The Museum of Modern Art, The California Academy of Sciences, and MIT to offer specialized content. For free. For everyone. Forever. #YouCanLearnAnything ... https://www.youtube.com/watch?v=bvcXEJbEzSs Note: All Khan Academy content is available for free at (www.khanacademy.org)