Luis and Julian take 2 days to assemble a toy plane. Julian and Riordan take 5 days to build the same plane while Luis and Riordan take only 2 days. How many complete days would the three of them take to build the same toy plane?
Let the plane be 1 P.

Fraction of the plane assembled by Luis and Julian per day
= ¹₂ P

Fraction of the plane assembled by Riordan and Julian per day
= ¹₅ P

Fraction of the plane assembled by Luis and Riordan per day
= ¹₂ P

Combined rate
= (¹₂ + ¹₅ + ¹₂) ÷ 2
= (⁵₁₀ + ²₁₀ + ⁵₁₀) ÷ 2
= ¹²₁₀  ÷ 2
= ¹²₁₀  x ¹₂
= ³₅ P 

Number of days taken by the three of them to build the same toy plane
= 1 ÷ ³₅
= 1 x ⁵₃
= 1.67

Since the number of days needed is 1.67 days, therefore the total number of complete days needed to build the toy plane is 2.

Answer(s): 2 days