How many neutrons does element x have if its atomic number is 23 and its mass number is 87?

100

The atomic mass of an element can be thought of as the number of protons + the number of neutrons. The atomic number of an element equals the amount of protons it has. Using this, we see that the number of neutrons can be calculated by subtracting the atomic number from the atomic mass. In this case, we get
153
−
53
=
100
.

The periodic table, surprisingly, carries much more info than most would think. Each item is in a specific place that accurately puts it into several precise categories at once, which scientists utilize constantly. 

In this case, one of those categories is the atomic number and mass number. These together hod the key to finding out the composition of the nucleus. 
The atomic number in elements is how many protons are in the nucleus. As you should know, a nucleus contains BOTH protons and neutrons. This is what is represented by the MASS number, which is protons +neutrons. (ignoring anything after the decimal point)

So, if Element has an atomic number (protons) of 23, and a mass number of 87, this means 23+neutrons=87. Do the inverse. 87-23=64
This means element X has 23 protons, 64 neutrons, and 23 electrons (assuming it's electrically neutral). 

The answer of neutrons is 64
Hope this helps!