To solve this problem, we can set up an equation, letting the unknown value of last month's supply sales be represented by the variable x.
First, we must convert 78% to its decimal equivalent, by dividing 78/100. This is because percentages are parts of a whole 100 percent.
78/100 = 0.78
Now, we are ready to set up our equation, using our knowledge that the word "of" refers to multiplication in mathematics.
3,285 = 0.78x
To solve our equation, we must divide both sides of the equation by 0.78 to get the variable x alone on the right side of the equation.
x = 4211.53846154
This means that last month's supply sales were about $4,211.54 (we round the last digit up because 8 is greater than 5).
Hope this helps!