Answer:
4 years old.
Step-by-step explanation:
Lets say that Josh's age is j and Kim's age is k. As Josh is 6 years older than 4 times Kim's age, we can write this as an equation:
j = 4k + 6
This is, John's age is equal to four times Kim's plus 6 years.
We then know that, when summed their ages is less than 31, so:
k + j < 31
Replace John's age by our first equation:
k + 4k + 6 < 31
5k + 6 < 31
Subtract 6 in both sides:
5k + 6 - 6 < 31 - 6
5k < 25
Dividing both sides by 5:
5k/5 < 25/5
k < 5
So, Kim's age MUST be less than 5.
If we talk about discrete values, this is 1, 2, 3, 4, 5, 6...., we will say than the oldest Kim could be is 4. If Kim is 5 years old, the sum of ages is 31 and we need it ti be LESS than 31. (we are not considering fractional numbers as 4.5 years old).
So, Kim can be, at maximum, 4 years old.
