int main()
{
     int a=7, t=0;
     t=--a+--a+a+++a;
     printf("%d",t);
}

output: 20

I could not understand why it prints '20' ? please explain it !

I'm a student of C myself. So please don't take my answer as an educated one.
However, that program is very unclear (maybe even so for the compiler).
Basically the expression goes:
t=(--a)+(--a)... and this is where it gets unclear. Either the rest of the expression goes t=(--a)+(--a)+a+(++a) or t=(--a)+(--a)+(a++)+a. Both of which would have different answers technically. The compiler maybe has a default or prioritized way of reading that expression.

That expression is calculated as follows:

t=(--a)+(--a)+(a++)+a;

First a gets decreased by 1 twice (--a) BEFORE they are added together

(a++)+a means that a will be increased by 1 AFTER it has been added with a again. And since a is never used again after this it doesn't matter that a=6 now.

Hence we get:
5+5+5+5=20