Solutions sent it at the yahoogroups were more or less spot on, so I am just replicating that here
1. How many pairs of integers (x,y) exist such that the product of x, y and HCF (x,y) = 1080?
We need to find ordered pairs (x, y) such that xy*HCF(x, y) = 1080.
Let x = ha and y = hb where h = HCF(x, y) => HCF(a, b) = 1.
So h^3(ab) = 1080 = (2^3)(3^3)(5).
We need to write 1080 as a product of a perfect cube and another number.
Four cases:
I h = 1, ab = 1080 and b are co-prime. We gave 4 pairs of 8 ordered pairs (1,1080), (8, 135), (27,40) and (5,216) (Essentially we are finding co-prime a,b such that a*b = 1080)
II h = 2, We need to find number of ways of writing (3^3) * (5) as a product of two coprime numbers. This can be done in two ways - 1 and (3^3) * (5) , (3^3) and (5)
number of pairs = 2, number of ordered pairs = 4
III - h = 3, number of pairs = 2, number of ordered pairs = 4
IV - h = 6, number of pairs = 1, number of ordered pairs = 2
Hence total pairs of (x, y) = 9, total number of ordered pairs = 18.
The pairs are (1,1080), (8, 135), (27,40), (5,216), (2,270), (10, 54), (3,120), (24,15) and (6,30),
2. Find the smallest number that leaves a remainder of 4 on division by5, 5 on division by 6, 6 on division by 7, 7 on division by 8 and 8 on division by 9?
LCM(5, 6, 7, 8, 9) - 1 = 2519.
3. There are three numbers a,b, c such that HCF (a,b) = l, HCF(b,c) =m and HCF (c,a) = n. HCF(l,m) = HCF(l,n) = HCF(n,m) =1. Find LCM of a,b,c. (The answer can be "This cannot be determined").