How many LEDs must be combined into one light source to give a total of 3.8 W of visible-light output (comparable to the light output of a 100 W incandescent bulb) ?

This is an incomplete question. The complete question is - -

An individual white LED (light-emitting diode) has an efficiency of 20% and uses 1.0 W of electric power.

How many LEDs must be combined into one light source to give a total of 3.8W of visible-light output (comparable to the light output of a 100W incandescent bulb) ?

And the answer is - -

19 LEDs

Explanation:

The full form of LED is Light emitting diode.

It is given that the efficiency of the LED bulb is 20 %

1 LED uses power = 1 W

So the output power of 1 LED = 0.2 W

We need to find the power required to give a 3.8 W light.

Power required for 3.8 W = Number of LEDs required = (total required power / power required for 1 LED)

= 3.8 / 0.2

= 19

Therefore, the number of LEDs required is 19.