64 can be divided by: 1, 2, 4, 8, 16, 32, 64

48 can be divided by: 1, 2, 3, 4, 8, 12, 16, 24, 48

So, their largest common factor is 16. Then,

64:48 = (64 ÷ 16) : (48 ÷ 16) = 4:3