To simplify this expression, we can follow the order of operations (PEMDAS/BODMAS).

First, let's calculate the exponents:
3^5 = 3 * 3 * 3 * 3 * 3 = 243
10^2 = 10 * 10 = 100
5^2 = 5 * 5 = 25
27^2 = 27 * 27 = 729
3^5 = 3 * 3 * 3 * 3 * 3 = 243

Now, let's substitute these values back into the expression:
243 - 100 : 25 - 729 : 243

Next, let's perform the multiplications and divisions from left to right:
243 - 100 = 143
143 : 25 = 5.72
5.72 - 729 : 243

Finally, let's perform the remaining division and subtraction:
5.72 - 729 = -723.28

Therefore, the simplified expression is -723.28.