A dessertspoon is not a defined measure (unlike teaspoonfuls and tablespoonfuls), but it usually is taken to mean somewhere between them. Here's the capacity of various spoonfuls:-
� teaspoon = 1.25 ml
� teaspoon = 2.5 ml
1 teaspoon = 5.0 ml
1 tablespoon = 15 ml
I reckon the answer to your question would be that as a dessertspoon holds 10 ml (half way between 5 ml and 15 ml), then you would need 1.5 dessertspoons to make up a tablespoon (15/10).