How do a put a formula in a spreadsheet if I want to multiply 2 columns

We just purchased Libre Office and my old spread sheet does not work with Libre. I am trying to take column O and multiply it with column P and i want to total in column W. The numbers in O & P will never be the same.

Example: Amount times Unit price gives price per item, one row per item. Then total.

Cellwise:

=O5*P5 in Q5

This for, say, cells Q5 up to Q95

Then =SUM(Q5:Q95) in (say) W2

Or =SUMPRODUCT(O5:O95;P5:P95) in W2

This gives the total without the prices per item.

… You may read a bit in the Calc Guide available from here.