GCF of 40 and 48:
40
2 20
2 10
2 5
48
2 24
2 12
2 6
2 3
40 = 2 * 2 * 2 * 5
48 = 2 * 2 * 2 * 2 * 3
They have 2 * 2 * 2 is common
2 * 2 * 2 = 8
GCF of 40 and 48 = 8