Adrian and Beck builds a toy car in 10 hours.
Beck and Christopher builds a toy car in 6 hours.
Christopher and Adrian builds a toy car in 9 hours.
How long does it take for Adrian, Beck and Christopher to build a toy car together?
Fraction of the toy car built by Adrian and Beck in 1 hour = ¹₁₀ 
Fraction of the toy car built by Beck and Christopher in 1 hour = ¹₆ 
Fraction of the toy car built by Christopher and Adrian in 1 hour = ¹₉ 

Total fraction of the toy car built by Adrian and Beck and Christopher in 1 hour
= (¹₁₀ + ¹₆ + ¹₉) ÷ 2
= (⁹₉₀ + ¹⁵₉₀ + ¹⁰₉₀) ÷ 2
= ³⁴₉₀ ÷ 2
= ¹⁷₄₅ x ¹₂
= ¹⁷₉₀ 

Time taken for Adrian, Beck and Christopher to build a toy car together
= 1 ÷ ¹⁷₉₀
= 1 x ⁹⁰₁₇
= 5⁵₁₇ h

Answer(s): 5⁵₁₇ h or 5⁵₁₇ hours