let's say the numbers are "a" and "b".
now, let's say "b" is the larger one, so.. what's three times "a"? well, 3*a or 3a. what's 8 more than that? well, 3a + 8. Thus, b = 3a + 8
now, we know their sum is 48. thus
a + b = 48
a + (3a + 8) = 48 <------- solve for "a".