Given that a plane is flyig to metroc city which starts it's journey 1275 miles away from Metro city.
So that means total distance to be travelled by the plane = 1275 miles
We can use formula (distance = speed x time ) to find the distance travelled in each slot.
Given that the plane travels for "a" hours at 550 mph
then distance covered = speed * time = 550*a=550a
Given that the plane travels for "b" hours at 590 mph
then distance covered = speed * time = 590*b=590b
Then total distance travelled = 550a+590b
Given that total distance should be 1275 so both values will be equal
550a+590b=1275
Hence first choice is the final answer.