Chapter 1 - Foundations for Algebra - 1-8 An Introduction to Equations - Practice and Problem-Solving Exercises - Page 56: 37

c=2

We must think in our head what times 13 equals 26. In order to do this, we guess low, whole numbers that may be close to 26 when multiplied by 13. 13 times 1 is 13, which is close but a bit low. However, 13 times 2 equals 26, so we know that c=2.