Adriel and Billy builds a toy car in 7 hours.
Billy and Christopher builds a toy car in 10 hours.
Christopher and Adriel builds a toy car in 2 hours.
How long does it take for Adriel, Billy and Christopher to build a toy car together?
Fraction of the toy car built by Adriel and Billy in 1 hour = ¹₇ 
Fraction of the toy car built by Billy and Christopher in 1 hour = ¹₁₀ 
Fraction of the toy car built by Christopher and Adriel in 1 hour = ¹₂ 

Total fraction of the toy car built by Adriel and Billy and Christopher in 1 hour
= (¹₇ + ¹₁₀ + ¹₂) ÷ 2
= (¹⁰₇₀ + ⁷₇₀ + ³⁵₇₀) ÷ 2
= ⁵²₇₀ ÷ 2
= ²⁶₃₅ x ¹₂
= ¹³₃₅ 

Time taken for Adriel, Billy and Christopher to build a toy car together
= 1 ÷ ¹³₃₅
= 1 x ³⁵₁₃
= 2⁹₁₃ h

Answer(s): 2⁹₁₃ h or 2⁹₁₃ hours