Riordan and Jeremy take 4 days to assemble a toy train. Jeremy and Julian take 5 days to build the same train while Riordan and Julian take only 4 days. How many complete days would the three of them take to build the same toy train?
Let the train be 1 P.

Fraction of the train assembled by Riordan and Jeremy per day
= ¹₄ P

Fraction of the train assembled by Julian and Jeremy per day
= ¹₅ P

Fraction of the train assembled by Riordan and Julian per day
= ¹₄ P

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

Number of days taken by the three of them to build the same toy train
= 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 train is 3.

Answer(s): 3 days