Adriel and Billy builds a toy car in 3 hours.
Billy and Christopher builds a toy car in 4 hours.
Christopher and Adriel builds a toy car in 9 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 = ¹₃ 
Fraction of the toy car built by Billy and Christopher in 1 hour = ¹₄ 
Fraction of the toy car built by Christopher and Adriel in 1 hour = ¹₉ 

Total fraction of the toy car built by Adriel and Billy and Christopher in 1 hour
= (¹₃ + ¹₄ + ¹₉) ÷ 2
= (¹²₃₆ + ⁹₃₆ + ⁴₃₆) ÷ 2
= ²⁵₃₆ ÷ 2
= ²⁵₃₆ x ¹₂
= ²⁵₇₂ 

Time taken for Adriel, Billy and Christopher to build a toy car together
= 1 ÷ ²⁵₇₂
= 1 x ⁷²₂₅
= 2²²₂₅ h

Answer(s): 2²²₂₅ h or 2²²₂₅ hours