Since the tip is 15% of some cost x, we can calculate that.
Remember to write the percent as a decimal for the calculations
Think of the tip as the question 2.40 is 15% of what number?
2.40/x = 15/100
Multiply both sides by x
2.40 = 15x/100
2.40 = 0.15x
So the tip 2.40 = 0.15x
Divide both sides 0.15 to solve for x
And that makes the cost before the tip 2.40/0.15 = 16.00
$16.00
The total bill for his lunch would be the cost before the tip plus the tip
We can calculate the final bill as $16.00 + $2.40 = $18.40