Question

An electric heater uses 3/ 5

Answer 1

In this question, all the vital information's required to reach the solution is already given. Based on those information's the answer to the question can be easily reached. Let us first write down all the information's given.
Amount of power used by the electric heater in 1 hour = 3/5 watts
Then
Amount of power used by the electric heater in 1/2 hour = (3/5) * (1/2) watts
                                                                                          = (3 * 1)/(5 * 2) watts
                                                                                          = 3/10 watts
So 3/10 watts of power is required by the electric heater in 1/2 an hour. The correct option among all the options given in the question is option "a".

Answer 2

This question could not come from any school source.  If it did,
then you should always be suspicious of anything that comes
from that source.  The question contains a fundamental mistake,
and misleads the student.

"Power", measured in "watts", is a rate, not a quantity.  It tells
how fast you're using energy.  You don't use some watts this
morning and some more watts this afternoon.  That's like saying
that you'll drive some miles per hour today, and then you'll drive
some more miles per hour tomorrow.  It doesn't work that way.

The heater doesn't use up some amount of watts.  It uses up
some amount of energy.  "Watts" is the measure of how fast
or slowly it uses up the energy.

This question makes sense if you get rid of the 'watts' and call it
'pints of milk' instead.

           "My cats drink 3/5 pint of milk each hour. 
            How many pints of milk will they drink in 1/2 hour ?"

               (1/2 hour) x (3/5  pint/hour)

           =  ( 1/2  x  3/5)  (pint)

           =        3/10  pint .