A Binary formula quick reference can help in many cases, especially when we are dealing with IP addresses for first time. The more and more you use them the need to look into the table will become less and less.
2a * 2b = 2a+b. For example 22 * 28 = 210.
2a + 2a = 2 * 2a = 21+a. For example 211 + 211 = 212.
2a + 2a + 2a + 2a = 4 * 2a = 22 * 2a = 22+a. For example 210 + 210 + 210 + 210 = 212.