Alfred and Bernard builds a toy car in 9 hours.
Bernard and Charlie builds a toy car in 3 hours.
Charlie and Alfred builds a toy car in 5 hours.
How long does it take for Alfred, Bernard and Charlie to build a toy car together?
Fraction of the toy car built by Alfred and Bernard in 1 hour = ¹₉ 
Fraction of the toy car built by Bernard and Charlie in 1 hour = ¹₃ 
Fraction of the toy car built by Charlie and Alfred in 1 hour = ¹₅ 

Total fraction of the toy car built by Alfred and Bernard and Charlie in 1 hour
= (¹₉ + ¹₃ + ¹₅) ÷ 2
= (⁵₄₅ + ¹⁵₄₅ + ⁹₄₅) ÷ 2
= ²⁹₄₅ ÷ 2
= ²⁹₄₅ x ¹₂
= ²⁹₉₀ 

Time taken for Alfred, Bernard and Charlie to build a toy car together
= 1 ÷ ²⁹₉₀
= 1 x ⁹⁰₂₉
= 3³₂₉ h

Answer(s): 3³₂₉ h or 3³₂₉ hours