how to solve this without excel or solver? step by step explanation for solving by hand of NORMINV because i am not allowed to use excel or any other solver so how do i do it

Revenue from segment A, PA = $3.50 per cubic foot Revenue from segment B, PB = $2.00 per cubic foot Mean demand for segment A, DA = 3,000 cubic feet Standard deviation of demand for segment A, OA = 1,000 cubic feet Using Equation 16.4, the capacity to be reserved for segment A is given by CA = NORMINV ( 1 - PB/ PA, DA, CA) = NORMINV ( 1 - 2.00 / 3.50, 3,000, 1,000) = 2.820 cubic feet Thus, ToFrom should reserve 2,820 cubic feet of truck capacity for segment A when customers from this segment are willing to pay $3.50 per cubic foot. If the amount customers are willing to pay increases from $3.50 to $5.00, the reserved capacity should be increased to CA = NORMINV ( 1 - PB / PA, DA, JA) = NORMINV ( 1 - 2.00 /5.00, 3,000, 1,000) = 3.253 cubic feet