For getting the answer of the total number of vitamins (v) you need to multiply the whole number of bottles (b) with the amount of vitamins present in a one bottle (50 vitamins). So , the answer is v = 50 × b / v =50b..Don't get confused both are the same.
The long piece of the board is 100 cm and the shorter board is 20. you can solve this by dividing 120 by 6 since u need to have a piece that is 5x a piece which is a total of 6 pieces. Then multiply that number by 5 and that is your longest board.