You can set different equations then solve them in order to get the 2 numbers.
Let one number be x and the other y, so x+y=55 (if you split 55 into two x and y, their sum will be 55)
1/3 of one is 2/5 or the other, so 1/3 of x is equal to 2/5 of y.
Hence the two equations: x + y = 55
1x/3 = 2y/5
By solving the system, you get : x = 30 and y = 25