The efficiency of a device such as a lamp can be calculated using this equation:

efficiency = (useful energy transferred ÷ energy supplied) × 100

using the formula, calculate the efficiency of the lamp shown in the diagram above.

a) 7%
b) 25%
c) 70%
d) 75%

efficiency = (useful energy transferred ÷ energy supplied) × 100

It's easy to use this formula, but we have to know both the useful energy and the energy supplied.  The drawing doesn't tell us the useful energy, so we have to find a clever way to figure it out.  I see two ways to do it:

Way #1:

We all know about the law of conservation of energy.  So we know that the total energy coming out must be  250J, because that's how much energy is going in.  The wasted energy is 75J, so the rest of the 250J must be the useful energy . . . (250J - 75J) = 175J useful energy.

(useful energy) / (energy supplied) =  (175J) / (250J) = 70% efficiency

Way #2:

How much of the energy is wasted ? . . . 75J wasted

What percentage of the Input is that 75J ? . . . 75/250 = 30% wasted

30% of the input energy is wasted.  That leaves the other 70% to be useful energy.

prndulum needs thread, weight clock to time swings