Answer:
0.2 (15+x) = 5
Explanation:
Let 'x' represent cost of Darius’s dinner
Ximena’s dinner cost $15, that will be added to cost of Darius's lunch i.e 'x'. They left a 20% tip on the TOTAL cost of both their lunches that would be equal $5. So, the equation is 5=0.2(x+15). 20% (0.2) of the sum of Ximena and Darius lunches i.e 5 dollars.
Equation : 0.2 (15+x) = 5
 
(0.2) (15) + 0.2x = 5
 3 + 0.2x = 5
 0.2x = 2
 x = 2/0.20
 x = 10
Darius’s dinner cost $10
 
Verification : 
 
15+10=25
25 x 0.20 = 5