The purpose of a trial balance is to prove that the value of all the debit value balances equals the total of all the credit value balances. If the total of the debit column does not equal the total value of the credit column then this would show that there is an error in the nominal ledger accounts.
it said that t= 4 and because the t is after the 40 you would times 40 by 4 to get 160. Then you add them all up and there you go you got your velocity and speed.