Question from the Algorithmics - Fundamentals test

The function calcPGCD is used to calculate the Greatest Common Divisor of two numbers a and b (0<=b<=a and a=qb +r):

function calcPGCD(a, b) {
            r = a%b;
            If r != 0 {
                   a <-- b;
                   b <-- r;
                   
            display calcPGCD(a, b);
   } 
 }

What does this code display when a is 21 and b is 15?