A linear equation that models the situation is 1.4p + 2n = 40.
This equation represents the total daily revenue from the pens (1.4p) and notebooks (2n) and sets it equal to the target revenue of $40. The variables p and n represent the number of pens and notebooks sold, respectively. This equation can be used to determine the number of pens and notebooks that need to be sold in order to reach the target revenue of $40.