Two numbers are in the ratio 5 : 8 and their HCF is 13.
What is their LCM?

Let the numbers be 5$k$ and 8$k$. Since 5 and 8 are co-prime, HCF = $k=13$, so the numbers are 65 and 104. LCM = $13\times 5\times$ 8 = 520. Answering 40 forgets to multiply by the HCF; 6760 is the product $65\times 104$; 104 is just the larger number.