First we write equation for new freezer where x will be number of days operated and than equation for second freezer where x will have same function and we set those 2 equations to be equal. after that all we need to do is to solve for x
500 + 0.2*x = 20 + 0.5*x
480 = 0.3*x
x = 480/0.3 = 1600
The answer is 1600 days