I believe the pre-decrement (or increment) will happen entirely before the final calculation. The post-increment (or decrement) will happen after the calculation. Thus:
two decrements happen first and a=5 and the increment happens after the calculation;
so: 5 + 5 + 5 + 5 = 20.
The value of 'a' after the calculation is a=6.
If you instead did:
the value of t=25 (5+5+5+5+5) and a=7.