The H₂O molecule is made up of two kinds of atoms i.e hydrogen and oxygen. As shown there is '5' written on the left of the molecule which indicates the number of molecules, so there are 5 numbers of the water molecules.
One water molecule is composed of two hydrogen atoms which are represented as H₂, and one oxygen atom which is represented as O. So the total number of hydrogen atoms is 5 X 2 = 10.
And the total number of oxygen atoms is 5. So the total number of atoms present in 5 molecules of water is 10+5 =15. So there are 15 atoms present in 5 molecules of water.