The Setup: We are dealing with classic probability, which means we need to find the total number of possible outcomes (the entire lobby) and divide it by the specific outcomes we actually want (the VIPs). The overarching rules: our number is a 3-digit integer between 100 and 400, the digits must add up to exactly 10, and our winning condition is that the number is divisible by 4. Let our 3-digit number be represented as abc.
Step 1: Define the hundreds digit.
Since n lies between 100 and 400, the hundreds digit (a) is heavily restricted. It can only be 1,2, or 3. We will map out the possibilities for each scenario.
Step 2: Scout the 100s tier (a=1).
If the first digit is 1, then the remaining two digits must add up to 9 to satisfy the sum rule (1+b+c=10⟹b+c=9).
The possible pairs for (b,c) are:
(0,9),(1,8),(2,7),(3,6),(4,5),(5,4),(6,3),(7,2),(8,1),(9,0).
That gives us 10 total numbers in this tier.
Now, apply the divisibility rule for 4 (the last two digits must form a number divisible by 4). Looking at our pairs, only 36 and 72 pass the vibe check.
* Total numbers: 10
* Valid hits: 2 (which are 136 and 172)
Step 3: Scout the 200s tier (a=2).
If the first digit is 2, the remaining digits must sum to 8 (b+c=8).
The possible pairs are:
(0,8),(1,7),(2,6),(3,5),(4,4),(5,3),(6,2),(7,1),(8,0).
That is 9 total numbers. Checking the last two digits for multiples of 4, we keep 08,44, and 80.
* Total numbers: 9
* Valid hits: 3 (which are 208,244, and 280)
Step 4: Scout the 300s tier (a=3).
If the first digit is 3, the remaining digits must sum to 7 (b+c=7).
The possible pairs are:
(0,7),(1,6),(2,5),(3,4),(4,3),(5,2),(6,1),(7,0).
That gives us 8 total numbers. Sifting for multiples of 4 in those last two digits, only 16 and 52 survive.
* Total numbers: 8
* Valid hits: 2 (which are 316 and 352)
Step 5: Calculate the final probability.
Now we just tally up the final score.
Total sample space (every possible valid number) =10+9+8=27
Favorable outcomes (the ones divisible by 4) =2+3+2=7
The probability P is the ratio of favorable outcomes to the total sample space:
P=277
Final Answer: 277