Owen and Fabian take 4 days to assemble a toy car. Fabian and Justin take 4 days to build the same car while Owen and Justin take only 5 days. How many complete days would the three of them take to build the same toy car?
Let the car be 1 P.

Fraction of the car assembled by Owen and Fabian per day
= ¹₄ P

Fraction of the car assembled by Justin and Fabian per day
= ¹₄ P

Fraction of the car assembled by Owen and Justin per day
= ¹₅ P

Combined rate
= (¹₄ + ¹₄ + ¹₅) ÷ 2
= (⁵₂₀ + ⁵₂₀ + ⁴₂₀) ÷ 2
= ¹⁴₂₀  ÷ 2
= ¹⁴₂₀  x ¹₂
= ⁷₂₀ P 

Number of days taken by the three of them to build the same toy car
= 1 ÷ ⁷₂₀
= 1 x ²⁰₇
= 2.86

Since the number of days needed is 2.86 days, therefore the total number of complete days needed to build the toy car is 3.

Answer(s): 3 days