How many exam papers earn a grade of $90%$

Question

It is an exercise question (#19) in the book with image showing all details.

My answer is given below, & request vetting. $$\sum_{k=19}^{20} \binom{20}{x}\cdot 3^{(20-k)} $$ $$\binom{20}{19}\cdot 3^1 => 60$$

Answer 1

You almost got it, $(0.9 ) 20 = 18$, it should be

$$\sum_{k=\color{red}{18}}^{20} \binom{20}{x}\cdot 3^{(20-k)} $$