The first term a=18 and

the common difference d=15 1/2​−18=−2.5

the last term is −47

Let the last term be the nth term

We know that the nth term of the arithmetic progression is given by a+(n−1)d

Therefore, a+(n−1)d=−47

⟹18+(n−1)∗(−2.5)=−47

⟹18+2.5−2.5n=−47

⟹20.5−2.5n=−47

⟹2.5n=20.5+47

⟹n=67.5/2.5​

⟹n=27
Therefore, the number of terms in the given sequence is 27