While working on the multiplication problem 39 × 20, Marcus says this: Four times 20 is 80, so 40 times 20 is 800. Now take away 1, so the answer is 799. Is Marcus’s method correct or not? If it is not correct, write equations that incorporate Marcus’s work and that show why it’s correct. If Marcus’s reasoning is not correct, work with portions that are right to correct Marcus’s work, and write a string of equations that corresponds to your corrected method for calculating 39×20.