Why is the equation like this. Here is the explanation.
We first need to find the price difference between the two subscriptions, therefore we subtract the cheapest subscription price from the expensive.
We then divide that number by the price per text message and end up with the number of texts needed for the subscriptions to cost the same. (We add the parentheses because of the order of operations. We want to subtract first, therefore we need the parentheses)
Let’s calculate the individual steps
$30-$20 = $10
$10/$0.1 = 100
The total number of messages they need to send for the subscriptions to cost the same is 100.