420
76 x 6 = 456. To do this mentally, you could separate the digits and then add. (6x6 is 36, and 70x6 is 420.)
Factors of 490: 1, 2, 5, 7, 10, 14, 35, 49, 70, 98, 245, 490.Factors of 420: 1, 2, 3, 4, 5, 6, 7, 10, 12, 14, 15, 20, 21, 28, 30, 35, 42, 60, 70, 84, 105, 140, 210, 420.GCF (490, 420) = 70