Addition and subtraction - Subtract with two-digit numbers - Presentation

In Lesson 14 of a mathematics presentation, students revisit the concept of subtracting two two-digit numbers, particularly when this involves crossing the tens boundary. The lesson poses a question to students about what should be done when there are insufficient units to subtract from. This is an important aspect of subtraction that requires understanding the process of exchanging. The lesson encourages discussion and the use of a number line to visually aid the subtraction process. Students are prompted to consider whether subtracting the ones before the tens is possible, or if partitioning the ones to count back to the next ten before subtracting the tens might be a more effective strategy.

Several activities are outlined to practice these subtraction skills. In Activity 1, students use a number line to subtract numbers like 12 from 51 or 25 from 48, demonstrating the step-by-step process and arriving at the correct answers. In Activity 2, students face a more complex challenge when they can't subtract the ones directly and must consider different partitioning methods to facilitate subtraction. Examples given include 42 minus 15 and 36 minus 19, where students are guided through the process of partitioning and subtracting to find the correct answers. The lesson also includes reasoning exercises where students must deduce subtraction problems based on given answers, and independent work activities that encourage using a number line, partitioning the minuend differently, or employing Base 10 for column subtraction. The presentation concludes with a discussion on various subtraction methods and their efficiency.