A stack of 20 identical boxes is 140 centimeters tall. Stefan takes three boxes off the top. How tall is the stack now?
We know that 20 boxes are 140 centimeters tall. Stefan takes away three boxes from the top of the stack. Now the stack is 17 boxes tall. To work out how tall the stack is now, first we need to work out the height of one box. If 20 boxes are 140 centimeters tall, to find the height of one box, we need to divide 140 by 20.
We can use a known fact to answer this calculation. We know that 14 divided by two is seven. So 140 divided by 20 is also seven. We know that Stefan took three boxes off the top. To calculate the height of those boxes, we can multiply seven by three or three by seven, which is 21. Now, if we take away 21 from 140, that would tell us the height of the stack now. 140 take away 20 is 120 and 120 take away one is 119. The stack is 119 centimeters tall.
First, we calculated the height of one brick. We did this by dividing 140 by 20, which was seven. Then we calculated the height of the three boxes that Stefan had taken off the top. Three times seven is 21. We took 21 from 140. And that gave us our answer of 119 centimeters.