It takes the earth 1.0 year to travel around the sun. It takes Venus 0.615 years to orbit the sun. If the planets are in a straight line with the sun now, how long will it take before this is the case again?
Let x be the number of years it takes before the planets are in the straight line with the sun again.
Then x/(0.615) is the number of times that Venus travels around the sun, and
x/(1.0)=x is the number of times the ...
This shows how to determine the length of time between alignments of planets.