The GCF is 10.
It is likely easy to notice that 30, 40, and 50 are all multiples of 10.
Listing the other factors of each number can help narrow it down.
Factors of 30 include: 1, 2, 3, 5, 6, 10, 15, 30
Factors of 40 include: 1, 2, 4, 5, 8, 10, 20, 40
And factors of 50 include: 1, 2, 5, 10, 25, 50
They all have 1,2,5, and 10 in common. However 10 is the GREATEST common factor that 30, 40, and 50 share.
The greatest common factor is 10.