Old Question, but I find this question interesting and seems like it might be useful for future reference.
So, I am posting the solution. If it is inappropriate then it can be deleted. Thank You.
Solution:
Let x be the amount of 50% acidic solution in litres and y be the amount of 60% acidic solution.
Given,
x + y = 80 ------------------------ (1)
and 50x + 60y = 55 * 80 --------------------------- (2)
multiplying eq 1 with 50 and subtracting from eq (2), we get:
10y = 400
=> y = 10
and x = 80 - y = 40
So 40 litres of both solution should be added to produce 80L of 55% solution