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1 solution
Using trigonometry,we can calculate that the light waves need to travel a distance of 20 cm after the first reflection in order to achieve the second reflection.The question is a little weird as the light waves begin by hitting mirror B first before hitting A and this will need to minus the horizontal distance which is 10cm from the total distance which is 100.Plus,the light waves need to travel 20cm of horizontal distance before experiencing the next reflection.Thus the calculation will be 90 divided by 20 and we will get 4.5,which is equal to 4 as the 0.5 is equal to 10cm which the light waves just strike the mirror B the fifth time