During the first 90 km of his trip, 'X' drove at a speed of 60 km/h. In the next 150 km, he increased his speed by 'y%' compared to his initial speed. For the remaining 20% of the total journey, he drove at a speed of 120 km/h. If the average speed for the entire trip was recorded as 75 km/h, determine the value of 'y'.