Last Math Problem:
The difference between a two-digit number and the number obtained by interchanging the positions of its digits is 36. What is the difference between the two digits of that number?
Answer:
The difference between the two digits of that number is 4.
Explanation: Let the ten’s digit be x and the unit’s digit be y. According to the explanation above, (10x+y)-(10y+x) = 36
To solve: 10x-10y+y-x = -9y+9x = 9(x-y) = 36
This results in the solution: the difference between the two digits (x-y) is 4.
Today’s Problem:
In the middle of a round pool lies a beautiful water-lily. The water-lily doubles in size every day. After exactly 20 days the complete pool will be covered by the lily. After how many days will half of the pool be covered by the water lily?