An essay contains 420 sentences. 80 of these sentences contain typing?
80/420, of course. To simplify that, first divide numerator and denominator by the obvious common factor 10, turning the fraction into 8/42. As both numerator and denominator are even, you can divide them both by 2, turning the fraction into 4/21, which cant be further simplified, since 4 is only divisible by 2, which is not a factor of 21.