During a 4-month-long drought, the water level in a lake changed by

-3 1/2 feet. The water level changed at the same rate each month.

What is amount the water level changed in 1 month?

In 4 months, the water level changed in the lake by - 3.5 ft.

We want to know the change in one month knowing that the change is the same for all months.

To do that, we can simply do a cross multiplication as follows:

change per month = (1*-3.5) / 4 = - 0.875 ft