Let

denote the random variable for the weight of a swan. Then each swan in the sample of 36 selected by the farmer can be assigned a weight denoted by

, each independently and identically distributed with distribution

.
You want to find

Note that the left side is 36 times the average of the weights of the swans in the sample, i.e. the probability above is equivalent to

Recall that if

, then the sampling distribution

with

being the size of the sample.
Transforming to the standard normal distribution, you have

so that in this case,

and the probability is equivalent to

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with the more expensive light bulbs, the cost would be $155.52
with the less expensive light bulb, the cost would be $119.04
This means you would save $36.48
I think the answer is yly>0