You spend $20 total on tubes of paint and disposable brushes for an art project. tubes of paint cost $4.00 each and paintbrushes cost $0.50 each. you purchase twice as many brushes as tubes of paint. how many brushes and tubes of paint do you purchase?

You are buying 2 items so let's use two different variables to represent each:
p = number of tubes of paint
b = number of disposable brushes
cost = price per item × number of items
Equations from information given:
4p + 0.50b = 20 ← from the first 2 sentences
2p = b  ← if there are twice as many brushes as paint tubes you need 2 times paint tubes to equal number of brushes
Solve the problem by using substitution... 1b equals 2p so replace b with that:
4p + 0.50(2p) = 20
4p + 1p = 20   ← 0.50 is half so half of 2p is 1p
5p = 20 ← combine like terms
p = 4   ← divide both sides of the equation by 5
Recall the equation 2p = b replace p with 4
2(4) = b 
8 = b
ANSWER:
You purchased 4 tubes of paint and 8 disposable brushes.