We know that 15 elves represent the 20% of the total.
Since 20% is one-fifth, the total number of elves is
[tex]15\cdot 5 = 75[/tex]
Alternatively, solve the proportion
[tex]15\div x = 20\div 100[/tex]
which yields
[tex]x=\dfrac{1500}{20}=75[/tex]
