Prime factorization of 210=2×3×5×7 So. four digit numbers with combinations of the digits |1. 6, 5. 7} and {2, 3, 5, 7} and three digit numbers with combinations of digits {6, 5, 7} will have the product of their digits equal to 210. Combination of four digits {1, 6, 5, 7} =4!=4×3×2×1=24 Combination of four digits {2, 3, 5, 7}=4!=4×3×2×1=24 Combination of three digits {6, 5, 7} =3!=3×2×1=6 Hence, total combinations =24+24+6=54