A number decreased by fifteen is less than two.

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.