Hi, goshawk,
It's best to think of the various clues with equals signs as giving different algebraic expressions for the same answer. For example, in 21dn, the answer can be expressed as HO-T/E-L or as HO-TE+L. You can use the fact that these two expressions lead to the same answer if you substitute the correct values for H, O, T, E and L to work towards these values.
As HO-T/E-L=HO-TE+L, we can ignore the HO term on both sides of the equation and we have -T/E-L=-TE+L. Rearranging this gives TE-T/E=2L. As T has to be a multiple of E (if it isn't, then TE-T/E can't be a whole number, which is necessary as the right hand side of the equation is 2L), and E, L and T are all whole numbers between 1 and 26, the maximum value for 2L is 52, and it turns out that there are only 13 possible pairs of E and T that give 2L less than or equal to 52, with L a whole number.
Similarly, in 11ac, we can treat the two expressions as an algebraic equation, and divide both sides by A (which is legitimate as A cannot be zero) to give D(E+L+T)=D+(E+L)T. I then set up a spreadsheet to calculate the two sides of the equation for the 13 combinations of E, L and T that I had from 21dn and all possible values of D, and found that there was only one value of D and one combination of E, L and T for which the two sides of this equation were equal. This is sometimes called "brute computation", because it involves a lot of calculations. However, D must be a factor of (E+L)T, and using this fact will reduce the number of calculations considerably.
The point is that the equations will not hold with ANY substituted numbers, only with a limited set of combinations, and in this case there is only one combination of D, E, L and T that satisfies the equations in both 11ac and 21dn.