Answer:
D + 550 = X 
The band accounted for 25% of the budget.
Explanation:
Since Fairfield High spent D dollars for refreshments, decorations, and advertising for a dance, and in addition, they hired a band for $ 550, to write an expression that represents the cost of the band as a fraction of the total amount spent for the school dance and determine, if D is $ 1650, what percent of the budget did the band account for, the following calculations must be performed: 
 
D + 550 = X 
 
D = 1650 
 
1650 + 550 = X 
2200 = X 
 
2200 = 100 
550 = X 
550 x 100/2200 = X 
55,000 / 2,200 = X 
25 = X 
 
Therefore, the band accounted for 25% of the budget.