Answer : In the reaction of Ca + 

 --->Ca

, the coefficient of Ca and 

 will be the same which will be 1.
Explanation : As per the rules of writing a chemical equation and balancing it properly, the number of elements in reactants side should be equal to the number of elements generated in product side. Hence when we use 1 molecule of Ca and one molecule of 

 then it generates 1 molecule of Ca

.