We know that a machine can print 405 copies in 1 minute. We are looking for how many can be made in 35 minutes. 35 minutes is 35 times greater than 1 minute, because if you have 1 35 times, you get 35.
This means that there will be 35 times more copies made in 35 minutes than in 1 minute. We know that there are 405 copies made in 1 minute, thus, in 35 minutes there would be 35 * 405 copies made, which is the same as taking 405 and adding it to itself 35 times.
