10% of 400 is 40. There are several ways to calculate this:

• Method 1: Write 10% as 10/100 and multiply it by 400. This gives 40 as the answer.
• Method 2: Divide 400 by 100 to get 4, which is 1%. Then, multiply 4 by 10 to get 40, which is 10% of 400.
• Method 3: Use the formula "Amount Saved = Original Price x Discount in Percent / 100". In this case, the original price is 400 and the discount is 10%. Plugging these values into the formula gives "Amount Saved = 400 x 10 / 100 = 40". Therefore, 10% of 400 is $40.

Regardless of the method used, the answer is the same: 10% of 400 is 40.