The LCM of two numbers is 520 and their HCF is 13.  How many such pairs of the numbers are formed?

2

LCM × HCF = Product of two numbers i.e. A × B

LCM = HCF (product of other factors i.e. a × b)

520 = 13(a × b)

a × b = 40

1 × 40 ....... (i)

2 × 20 ....... (ii)

4 × 10 ....... (iii)

5 × 8 ........ (iv)

(ii) and (iii), both have common factor 2

Therefore, possible factors are (i) 1 × 40 and (iv) 5 × 8