a) is self explanatory. For b, as the lowest common multiple of the 2 numbers is 600, 24 only has 2 and 3, thus, m must have a factor of 5 in it. The highest common factor between m and 24 is 4, thus m must have the factor 2, but it must be 2^2, anything lower makes the HCF less than 4 and anything more makes it 8 or more.

Thus, m=2^2 × 5^2 (from 600, must have the 5^2 factor)=100

Little bit complicated, just practice. Hope you'll get better :)