The first number is 40. The second, third, and fourth number each add 7, so we have 3 steps to get to the fourth step, so we must add 7 *3 = 21 to 40, which is 61.
You just keep adding 7 to the first number. First number is 40. Second number is 40+7 which is 47. Third number is 47+7 which is 54. Fourth number is 54+7 which is 61.
Or you could do 7 X 3 = 21 and add that to 40. The answer is 61.