Let us assume number of n.
The number is decreased by 15.
Decreasing by 15 means, we need to subtract 15 from that number.
So, it would be come n-15.
Now, it is said that the number would become less than 2.
So, we need to setup an inequality:
n-15 is less than 2.
n-15 < 2 : required inequality .
Adding 15 on both sides, we get
n-15+15 < 2+15
n < 17.
So, we could say the number would be less than 17.