Mr Wong received his first-month bonus. He deposited half of the amount in the bank. He used 1 10 of the remaining bonus to buy a ring for his wife, and then he bought his friends 8 mangoes, worth $10 each. In the end, he had $100 left in his wallet. What was Mr Wong's first-month bonus?

To find Mr. Wong's first-month bonus, we work backwards from the given expenditures and remaining amount. After calculating the expenses for the ring and the mangoes and knowing he has $100 left, we find that his bonus was $400.

Explanation:

To determine Mr. Wong's first-month bonus, we need to work backwards from the information given. Let's denote his bonus as B.

First, Mr. Wong deposits half of it in the bank, so he's left with
B/2.
He then uses 1/10 of the remaining bonus to buy a ring, so he spends (B/2)*(1/10) = B/20 on the ring.
This deduction leaves him with B/2 - B/20. To combine these fractions, we need a common denominator, which would be 20, so it is (10B - B)/20 = 9B/20 after buying the ring.
Next, he buys 8 mangoes, each worth $10, totaling 8*$10 = $80, so he spends another $80, leaving him with 9B/20 - 80.
After all spending, he has $100 left in his wallet.
Therefore, 9B/20 - 80 = $100. To find the bonus B, we solve for B in the equation 9B/20 = 100 + 80, which gives us 9B/20 = 180.
After multiplying both sides by 20/9, we find that B = (180*20)/9, which simplifies to B = $400.

So, Mr. Wong's first-month bonus was $400.