Cans of drinks are packed in boxes, each containing 12 cans. The weights of these cans are normally distributed with mean 460 g and standard deviation 12 g. The weights of the boxes, when empty, are independently normally distributed with mean 200 g and standard deviation 8g.
Two cans are chosen at random. Calculate the probability that the weights of the two cans differ by less than 20 g.
From the booklet answer, it has specific that I need to use $\mathsf P(-20 < c_1 + c_2 < 20)$, but I can't get it why there is a negative 20 in front $c_1 + c_2$. Isn't it $\mathsf P(c_1 - c_2 < 20)$?