Adrian and Bobby builds a toy car in 5 hours.
Bobby and Christopher builds a toy car in 4 hours.
Christopher and Adrian builds a toy car in 10 hours.
How long does it take for Adrian, Bobby and Christopher to build a toy car together?
Fraction of the toy car built by Adrian and Bobby in 1 hour = ¹₅ 
Fraction of the toy car built by Bobby and Christopher in 1 hour = ¹₄ 
Fraction of the toy car built by Christopher and Adrian in 1 hour = ¹₁₀ 

Total fraction of the toy car built by Adrian and Bobby and Christopher in 1 hour
= (¹₅ + ¹₄ + ¹₁₀) ÷ 2
= (⁴₂₀ + ⁵₂₀ + ²₂₀) ÷ 2
= ¹¹₂₀ ÷ 2
= ¹¹₂₀ x ¹₂
= ¹¹₄₀ 

Time taken for Adrian, Bobby and Christopher to build a toy car together
= 1 ÷ ¹¹₄₀
= 1 x ⁴⁰₁₁
= 3⁷₁₁ h

Answer(s): 3⁷₁₁ h or 3⁷₁₁ hours