Given:
string of decorative lights = 28 ft long
1st light is 16 inches from the plug.
1st, convert feet to inches.
1 foot = 12 inches
28 ft * 12 inches/ft = 28 * 12 inches = 336 inches long
2nd, deduct the distance of the 1st light from the plug.
336 inches - 16 inches = 320
3rd, divide the remaining length by the distance of 4 inches per light
320 ÷ 4 inches per light = 80 lights
There are 80 lights on the string.