Here is another way of looking at it. It is a little more complex, but it allows us to set everything up as a single equation that needs to be solved for x.
It may be easier to think of these as test scores rather than dollars.
Overall test scores for four tests average 100. The drone test score is 70 points. What is the average score of the remaining tests?
The drone counts for ¼ of the overall test score, and the remaining three tests count for ¾ of the overall average. If x = average of the three other test scores we get:
¼×70 + ¾ x = 100. Solving for x will give us x=110.
The advantage of this approach is that it is rather flexible and permits the solution of more complex problems.
For example, suppose we are told that the drone price was 80% of the average of the other three stocks and that the overall average is 110. What is the price of the drone stock?
Again let x = average price of the other three stocks. The drone stock price is .8 x.
We get ¼ .8x + ¾ x = 110. Solve for x. The drone price would be .8x.