Write and solve an equation for this problem. First represent the number of each item and the total cost for each item.
Kawan buys some protein bars at $0.80 each and bottled water at $0.90 each for a group hike. He buys twice as many bottles of water as protein bars. If he spends $52 in total, how many bottles of water and protein bars does he buy?
Let x = the number of protein bars