Question

The guests enjoyed their dinner and then Liz brought in a cake. The cake __________________(DECORATE) with chocolates and jellies, and it had as many candles as the number of years Carlos had reached that day. Carlos couldn’t blow them out on the __________________(ONE) try and his friends were delighted to help him.

Answer 1

The cake was decorated (Торт был украшен - пассивный залог, используем формулу was/were + глагол с окончанием -ed, так как decorate правильный глагол). 
on the first try (the first - используем порядковое числительное, так как он не задул свечи в первый раз). 

Answer 2

The cake was decorated with chocolates and jellies, and it had as many candles as the number of years Carlos had reached that day. Carlos couldn’t blow them out on the first try, and his friends were delighted to help him.

So, the complete sentence would be:

"The guests enjoyed their dinner and then Liz brought in a cake. The cake was decorated with chocolates and jellies, and it had as many candles as the number of years Carlos had reached that day. Carlos couldn’t blow them out on the first try, and his friends were delighted to help him."