Seven less than a number is twenty (as an equation)

You're expression is correct if it's asking for a number that is less than 7. But if the problem is asking for some number less than seven as in seven less some number, then it's 7-n. For example: 4 less than 7 is 7-4.

Hope this helps!

C. 1/3 of an hour

Step-by-step explanation: 5 min + 5min +5min =15 min + 5 = 20 Min