Answer
The speed of the San Francisco plane is $250$ mph,
The speed of the San Diego plane is $200$ mph.
Work Step by Step
Let the speed of the San Francisco plane be $x$ mph.
Distance traveled by either plane = (average speed)(time of travel).
In half an hour,
the San Diego plane covers $(x-50)\displaystyle \cdot\frac{1}{2}$ miles,
the San Francisco plane covers $ x\displaystyle \cdot\frac{1}{2}$ miles.
The sum of the two distances from LA is given as (equal to) 275 miles.
We solve:
$(x-50)\displaystyle \cdot\frac{1}{2}+x\cdot\frac{1}{2}=275\qquad$ ... multiply both sides by 2
$ x-50+x=550\qquad$ ... subtract 50
$2x=500$
$x=250$ mph
The speed of the San Francisco plane is $250$ mph,
The speed of the San Diego plane is $250-50=200$ mph.
