Let’s denote the first term of the arithmetic progression (A.P.) by a and the common difference by d. Then, the fourth term is a + 3d and the tenth term is a + 9d. According to the information given in the problem, we have the following system of equations:

a + 3d = 13……eqn 1

a + 9d = 31…….eqn 2

Subtracting the first equation from the second, we get 6d = 18, which implies that d = 3. Substituting this value of d into the first equation, we get a + 9 = 13, which implies that a = 4.

Now that we know the values of a and d, we can find the value of any term of the A.P. In particular, the 21st term is given by a + 20d = 4 + 20 * 3 = 64.

So, the value of the 21st term of the A.P. is 64.