Question

Of 100 clock radios with digital tuners and / or CD players sold recently in a department store, 70 had digital; tuners and 90 and CD players. How many radios had both digital tuners and CD players?

Answer 1

The answer is 60.
100 = 70 + 90 - DTandCD

Answer 2

I do not get how that come to 60

Answer 3

I wrote the formula that shows how I got it. What don't you get about it?