It is pretty straightforward once you remember some basic facts relating to representation of numbers in different bases (radixes.)

Some definitons, first:

S[color=#FF0000] := [/color]D means S is defined as D;

R^N == R * R * R … * R (R multiplied by itself N times)

R^0 == 1 (because R^0 = R/R = R^1*R^-1 = R^(1-1))

In the decimal number system, with base 10 (R=10), we use the ten symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 to write our numbers in base-10 notation.

Take the decimal number 12357 as an example; it has five digits (D=5)

12357 (in decimal) [color=#FF0000]:=[/color] 1 * 10^4 + 2 * 10^3 + 3 * 10^2 + 5 * 10^1 + 7 * 10^0 (in decimal)

In binary number system, with base 2 (R=2), we use only the symbols 0 & 1 to write our numbers in base-2 notation.

Now, take the binary number 110011 as an example; it has six digits (D=6).

110011 (in base-2) [color=#FF0000]:=[/color] 1 * 2^5 + 1 * 2^4 + 0 * 2^3 + 0 * 2^2 + 1 * 2^1 + 1 * 2^0 (in decimal)

In the hexadecimal number system, with base 16 (R=16), we have the sixteen symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F to write our numbers in base-16 (hex) notation (with t(0) = 0, t(1) = 1, …, t(A)=10, t(B)=11, …, t(F)=15; where t is a function that translates a symbol in the base-16 system to a number in the decimal system.)

Now, consider the hexadecimal number FF0123AB as an example; it has eight digits (D=8):

FF0123AB (in hex) [color=#FF0000]:=[/color] t(F) * 16^7 + t(F) * 16^6 + t(0) * 16 ^ 5 + t(1) * 16 ^ 4 + t(2) * 16^3 + t(3) * 16^2 + t(A) * 16^1 + t(B) * 16^0 (in decimal)

Replacing each f(.) above with the the corresponding number we get:

FF0123AB (in hex) [color=#FF0000]:=[/color] 15 * 16^7 + 15 * 16^6 + 0 * 16 ^ 5 + 1 * 16 ^ 4 + 2 * 16^3 + 3 * 16^2 + 10 * 16^1 + 11 * 16^0 (in decimal)

To generalize, consider the base-R number system; we use the R symbols s0, s1, s2, …, s (R-2), s(R-1) to write our numbers in this base (assuming t (s0) = 0, t (s1) = 1, t (s2) = 2, …, t (s(R-2)) = (R-2), t (s(R-1)) = (R-1); where t is a function that translates a symbol in the base-R system to a number in the decimal system.)

Now consider a number in this base: zxy … cba, with 26 digits (N=26) (where each digit is one of the R symbols s0, s1, s2, …, s (R-2), s(R-1))

Then zxy … cba (in base-R) [color=#FF0000]:=[/color] t(z) * R^(N-1) + t (x) * R^(N-2) + t(y) * R^(N-3) + … + t© * T^2 + t(b) * R^1 + t(a) * R^0 (in decimal)

(Replace each f(.) above with the the corresponding number to get the concrete answer.)

Now armed with this knowledge, you are ready to answer your own question.

You know what, here a is challenge for you: write a program that converts numbers represented in one base to corresponding numbers in another base (hint: use the decimal number system as the intermediate system.)