One quick physical science question

1. why isn’t it possible for an atom to have a mass number of 15 and an atomic number of 18?

Click an item in the list or group of pictures at the bottom of the problem and, holding the button down, drag it into the correct position in the answer box. release your mouse button when the item is place. if you change your mind, drag the item to the trashcan. click the trashcan to clear all your answers. label the following topographic map. click on a label below the map to select it, and then click on the appropriate box on the map to place the label. (increments for contour lines is 50 ft).

What is responsible for the uneven heating between the poles and the equator on any given day

Which step is not included in the step approach to calculating the greatest common divisor?