A man has ₹15,000. He lends a part of it to X for 3 years at 10% simple interest and the remaining to Y for 3 years at 12% simple interest. If the total interest received from both X and Y after 3 years is ₹4,950, find the amount lent to X.
- (a)₹7,000
- (b)₹7,500
- (c)₹8,000
- (d)₹8,500
Answer
Why
Correct — B. Both loans run for the same 3 years, so work with one year's interest.
Interest per year: 4,950 ÷ 3 = ₹1,650
Let X get ₹x at 10%, and Y get ₹(15,000 − x) at 12%
Interest: 0.10x + 0.12(15,000 − x) = 1,650
Expand: 0.10x + 1,800 − 0.12x = 1,650
Collect: 0.02x = 1,800 − 1,650 = 150
Divide by 0.02: x = ₹7,500 → option (b)
Why the others are wrong
- (a)₹7,000 — ₹7,000 to X leaves ₹8,000 to Y: 2,100 + 2,880 = ₹4,980 over 3 years, ₹30 more than ₹4,950.
- (c)₹8,000 — ₹8,000 to X leaves ₹7,000 to Y: 2,400 + 2,520 = ₹4,920 over 3 years, ₹30 short of ₹4,950.
- (d)₹8,500 — ₹8,500 to X leaves ₹6,500 to Y: 2,550 + 2,340 = ₹4,890 over 3 years, ₹60 short of ₹4,950.
Concept
The yearly interest fixes the average rate: 1,650 ÷ 15,000 = 11%.
11% sits exactly halfway between 10% and 12%, so by alligation the two parts are in the ratio (12 − 11) : (11 − 10) = 1 : 1. X and Y get ₹7,500 each.
The 3-year figures give the same answer. X's part earns 30% and Y's 36% over the period, and 0.30x + 0.36(15,000 − x) = 4,950 gives x = 7,500.
Key facts
- Simple interest = principal × rate × time ⁄ 100.
- When both parts run for the same time, divide the total interest by the time first.
- When the average rate is midway between the two rates, the two parts are equal.
Study next
Common traps
- Treating ₹4,950 as one year's interest: 0.10x + 0.12(15,000 − x) = 4,950 gives a negative x, a sign the 3 years were left out.
- Splitting the money 10 : 12 like the rates. The split comes from the distances to the 11% average, which are equal.
17 Sep 2025, 16:00, Quant Q.7 splits ₹5000 between 6% and 9% for one year: the 7.8% average gives 1.2 : 1.8 = 2 : 3, so ₹2000 is at 6%.
18 Sep 2025, 12:30, Quant Q.8 gives the difference of the interests instead: over 4 years, 0.40y − 0.32(20,000 − y) = 2,240, so 0.72y = 8,640 and the 10% part is ₹12,000.
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