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Purchase items to solve your mana issues Early game:
You can get Meki Pendant 390 gold
So you can work on the Tear of the Goddess

Or get x2 Faerie Charm 180 gold each
An work on this Mana Manipulator
It will help you and your lane partner with mana for only 475 gold total.

Or if you want to save some extra gold for this Eleisa's Miracle 1,200 gold

Just make sure your working on Tear of the Goddess
After buying Mana Manipulator or Eleisa's Miracle

Early Game
Early game you want to focus on last hits minions and farm.

[Purchase order Early Game]

Tear of the Goddess
Berserker's Greaves
The Brutalizer
Cost in total: 3,252 gold

Once you get those items above get blue / red and farm up on minion and kills.

Mid Game
At this point you should have enough to get 2 upgrades before going back to farm / kills.

[Purchase order Mid Game]
Buy: Sheen

Then turn that The Brutalizer into a Youmuu's Ghostblade
Cost in total: 2,687 gold

Once you get those items above get blue / red and farm up on minion and kills.
Save up 2,740 gold so you can turn that Sheen Into a Trinity Force

So Mid game you should have already spent 5,427 gold

Late Game
At this point the game hasn't ended. Just farm jungle, minions and get blue with afew more kills.

[Purchase order Late Game]
Buy this.
Giant's Belt
Cost: 1,110 gold

[Upgrade this]

Turn this Tear of the Goddess Into this Manamune
Total Gold Spent: 2,225 gold.

If you have some extra gold turn that Giant's Belt Into Warmog's Armor
For 1,890 more gold.

Later on buy which ever you like.

Atma's Impaler
Banshee's Veil
Guardian Angel