We know from the givens that "Their highest selling month is 325% of their lowest selling month.".
Transforming this into an equation, we get:
highest selling month = 3.25 x lowest selling month
We are also given the amount in the lowest selling month as 980 units.So, all we need to do is substitute in the equation to get the number of units in the highest selling month as follows:
highest selling month = 3.25 x 980 = 3185 units