540 seconds, because this is just a proportion. You find the number of seconds in 15 mins, which is 900 seconds, then you do 85 blinks/900 seconds = 51 blinks/? seconds. You cross multiply and get 45,900=85x. You divide each by 85 to get x=540.
1 min = 60 seconds, so 15 min = 15 x 60 seconds = 900 seconds 85 blinks in 900 seconds 900/85 = 10.58 seconds per one blink (this number is rounded, but final answer is exact) 10.58 x 51 = 540 seconds for 51 blinks
