When you've got Google.
I admit I still use the same calculator I bought for my high school algebra one class. But it turns out I don't need it. I just found through a rather round about path that if you enter any mathematical formula into Google search it will give you the answer.
So for example if you wanted to know what (4(57/17)+15^3.5)/2 was equal to, you could simply type it into search.
Not only that but you can also convert feet to meters or get a dictionary definition, that's pretty neat.
Interestingly if you search for the answer to life the universe and everything, it also gives you the correct answer.