First we need to find the GCF (greatest common factor)
Factors of 42 are:
1, 2, 3, 6, 7, 14, 21, 42
Factors of 48 are:
1, 2, 3, 4, 6, 8, 12, 16, 24, 48
GCF is 6
To simplify you divide numerator and denominator into the GCF
42 ÷ 6 = 7
48 ÷ 6 = 8
42/48 = 7/8
Hope this helps. :)