Answer:
C. 20 minutes
Explanation:
If Victor (V) worked alone, it would take him 30 minutes.
30 * V = 1
V = 1 / 30
When x represents the number of minutes it would take Dennis (D) working alone,
x * D = 1
D = 1 / x
Working together, they would finish in 12 minutes
12 (V + D) = 1
12( 1/30 + 1/x) =1
1/30 + 1/x = 1/12
Multiply all through by x
x/30 + 1 = x/12
x/30 - x/12 = - 1
( 2x - 5x ) / 60 = - 1
- 3x / 60 = - 1
- 3x = -60
x = -60 / -3 = 20 minutes