Assuming the distribution is continuous, you have

If instead the distribution is discrete, the value will depend on how the interval of number between 1 and 29 are chosen - are they integers? evenly spaced rationals? etc
Answer: 2/3
Cathy rolls a number cube that is labeled from 1 to 6.
Total outcomes = {1,2,3,4,5,6}
she rolls an even number or a multiple of 3
Outcomes of an even number = {2,4,6}
Outcomes of multiple of 3 = {3,6}
so favorable outcomes of even number or a multiple of 3 = {2,3,4,6}
Probability ( any event) =number of favorable outcomes divide by total number of outcomes
probability(even number or a multiple of 3) = 
Ivan forgot to follow PEMDAS
Division first
12/3 = 4
4 x 2 = 8
8 + 8 = 16
Hope this helps!
Answer: He made $90 last week.
Assumption :
Let, Luis made $X last week.
He made $72 this week which is 80% of $X that he made last week
⇒ X * 80% = 72
⇒ X * 80/100 = 72
⇒ X = (72 * 100)/80
⇒ X = 90
Therefore, he made $90 last week.
To remember :
While solving this type of problems, be careful to understand which one of last time and current time is not given; just assume that one and input other conditions, the problem will be solved.
Hey there,
solving for a is just like you think of all other letter as a constant
i.e the only variable is a
so you need to subtract b as a start
d=a+b-c/3
-b -b
=============
-b+d=a-c/3 now it's time to add c/3 (isolate a by itself in one side)
-b+d=a-c/3
+c/3 +c/3
==============
c/3-b+d=a
Hence you have solved for a