Final answer:
The number of electric-field lines coming out of a charge is proportional to the charge's magnitude, so a +20 nC charge has four times the field lines as a +5 nC charge, giving us 20 field lines.
Step-by-step explanation:
The number of electric-field lines coming out of a charge is proportional to the magnitude of that charge. Since five electric-field lines are associated with a +5 nC charge, a +20 nC charge has four times the magnitude. Thus, it should have four times the number of electric-field lines. Therefore, the answer to the question 'If five electric-field lines come out of a +5 nC charge, how many electric-field lines should come out of a +20 nC charge?' is: