When will 46 min and 69 min start at the same time again

You need to find the lowest common multiple of the two numbers by writing them as the product of their prime numbers so

46 = 2x23

69 = 3x23

Then you multiply these together, but as 23 occurs twice you only use one

23x2x3 = 138

so they will start at the same time 138 minutes later (2 hours 18 mins)