1 ppm = 1 part/1,00,000 parts.
So you convert 17 mg/375g to g/10^6 g.
17 mg = 0.017 g
The easiest way to do this I think is to multiply numerator and denominator by some numbers such that 375 becomes 1,000,00. That's asking 375g x ? = 1,000,000g and calculate ? as 1,000,000/375 = 2,666.7
Now 0.017g x 2,666.7 = approximately 45.
Another way of doing it, and it may be a little clearer this way, is
17 mg/375g = 0.017g/375g = 4.53E-5gPb/1 gH2O. Multiply both numerator and denominator by 1E6 to convert to g/1,000g or ppm. Then 4.53E-5*1E6/1E6 = 45.3 ppm.