For step one from 90 to 85, you are decreasing by 5. Then from step two, you are decreasing 85-75 which is 10 (which is 5*2). Step three is 75-60 = decrease of 15 (which. Is 5*3), and step four is a decrease of 60-40=20 (which is 5*4).
Therefore, the next number in the series would be step 5, and so 40-(5*5) gives the value, following the rule.
40-25= 15
The next number in the series is 15! :))