Camille had $18.90 to spend on groceries. After buying 10 grapefruits, she had $3.90 left. If each grapefruit costs the same amount, what is the price of one grapefruit?
If Camille had $3.90 left after buying 10 grapefruits, then she spent $18.90 - $3.90 = $<<18.9-3.9=15>>15 on grapefruits.
The price of one grapefruit is $15 / 10 grapefruits = $<<15/10=1.50>>1.50 per grapefruit. Answer: \boxed{1.50}.