Hey there!
27 tells us the sum of protons and neutrons is 27.
Al tells us we have 13 protons.
3- tells us that there are 3 more electrons than protons.
13 + n = 27
neutrons = 14
13 + 3 = 16 electrons
27Al3- has 13 protons, 14 neutrons, and 16 electrons.
Hope this helps!
