Well if you assume the function is piecewise linear you can make a nested if statement in Excel that should do what you want.
Note I just threw this together you are free to use it, but test it and make sure it is giving the values you wish.
Here is the function.
This assumes that the data is is the cell A1.Code:=IF(A1>70,A1,IF(A1>60,(1/2)*(A1-60)+65, IF(A1>50,(1/2)*(A1-50)+60,IF(A1>30,(1/4)*(A1-30)+55,6*A1))))
I hope this helps.