A half hour is 30 minutes. So you would need to divide 30 by 5 and you get six. Multiple 6 degrees Fahrenheit by 6 and you get 36 degrees. The solution cooled 36 degrees in half an hour.
Hope this helps!
Counting by Tens with numbers
10, 20, 30, 40, 50, 60, 70, 80, 90
Counting by Tens with words
ten, twenty, thirty, forty, fifty, sixty, seventy, eighty, ninety, one hundred
Number Patterns when counting by Tens
When you count by tens the numbers create a pattern. All the numbers end with a zero. The first digits are just like the numbers when you count (1, 2, 3, 4, 5, etc.). This pattern gives the numbers 10, 20, 30, 40, 50, etc.
found from: http://www.aaamath.com/k4c_cox1.htm
X=15 because 2(15) is 30-5 is 15
X/4 + x/5 = 1
5x/20 + 4x/20=1
9x/20 =1
9x = 20
x = 2.22 hours