The result after adding 3.8×10−6  to  4.2×10−5 with due regard to significant figures is

3.8×10−6+4.2×10−5=(3.8×10−1+4.2)×10−5=(0.38+4.2)×10−5=(4.58)×10−5Rounding off to one place of decimal.The sum =4.6×10−5