Three planets are in line and are orbiting round a sun, one take 1 week to orbit the sun another takes 9 weeks and the other takes 27 weeks. When is the next time they will all be in alignment?

27 weeks.

Step-by-step explanation:

You need to calculate the minimun common multiple between the three times.

1 week = 1^1 week

9 weeks = 3^2 weeks

27 weeks = 3^3 weeks

You take the 1 and the 3 with the biggest exponent and multiply. In this case is 1^1*3^3 = 27. So, in 27 weeks the pkanets will all be in alignment.