Answer:
Let the radii of two cylinders be 4x and 5x and their heights be 7y and 6y respectively. The lateral surface area of a cylinder is given by the formula 2πrh where r is the radius and h is the height. The lateral surface area of the first cylinder is 2π(4x)(7y) = 56πxy. The lateral surface area of the second cylinder is 2π(5x)(6y) = 60πxy. Therefore, the ratio of their lateral surface area is 56πxy : 60πxy which can be simplified to 14:15. Hence, the ratio of their lateral surface area is 14:15.