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jmc

number theory senior

Problem

There are two cameras that take pictures of a traffic intersection. Camera A starts taking pictures at AM and takes a picture every minutes. Camera B starts taking pictures at AM and takes pictures every minutes. Camera A and Camera B take a picture at the same time at four different times before noon. When Camera A and Camera B take their last picture together, how many minutes before noon is it?
Solution
If Camera A and Camera B take a picture at the same time, they will take a picture minutes later at the same time. Therefore, if we can find the first time they take a picture together, then we can continue adding minutes to figure out when the fourth picture was taken. Camera A's first pictures after AM is at , followed by and . Camera B will take a picture at . From here, we add minutes until we have taken four pictures. is followed by , which is followed by , which is followed by . This is minutes before noon.
Final answer
41