If x varies inversely as y and x = 2 when y = 6, then the value of y when x = 3 is:
- (a)3
- (b)4
- (c)2
- (d)6
Answer
Why
Correct — B. 'Varies inversely' fixes the product, not the ratio: x ∝ 1/y means xy = k.
Find k from the pair you are given:
k = 2 × 6 = 12
Now put x = 3 into xy = 12:
y = 12 ÷ 3 = 4 → option (b)
Direction check: x rose from 2 to 3, so y must fall. Anything at or above 6 is wrong before you calculate.
Why the others are wrong
- (a)3 — 3 is the new value of x written into the answer slot. The question asks what y becomes once x reaches 3, which is 12 ÷ 3.
- (c)2 — 2 is x's starting value. Nothing in the problem sends y to a number x once held — the invariant is the product xy = 12, not either variable on its own.
- (d)6 — 6 is y left unchanged. With x up from 2 to 3, y has to fall; holding y at 6 would make the product 18 instead of the fixed 12.
Concept
Two quantities vary inversely when their product is constant: x = k/y, so x₁y₁ = x₂y₂.
Direct variation fixes the ratio instead: x₁/y₁ = x₂/y₂. One word in the question — 'inversely' — decides which setup you write, and the rest is a single constant.
The same structure runs through most arithmetic word problems: men and days, speed and time over a fixed distance, pipes and hours. More of one means proportionally less of the other, with the product standing for the fixed job.
Key facts
- Inverse variation means the product is constant: xy = k, so x₁y₁ = x₂y₂.
- Here 2 × 6 = 3 × y, which gives y = 4.
- Direct variation on the same numbers would fix x/y = 1/3 and give y = 9 at x = 3.
- Multipliers are reciprocal: x multiplied by 3/2 forces y to be multiplied by 2/3.
Study next
Common traps
- Setting up 2/6 = 3/y, the direct-variation proportion, which returns 9.
- Answering with the value of x that was just substituted.
- Reading 'inversely' as swapping the two numbers rather than as fixing their product.
Tier-I keeps this to one line. The harder versions replace y with y² or √y, so the constant becomes xy² or x√y — the product rule is unchanged, only the power moves.
Quant Q.25 of this shift (09 Sep 2024, 12:30) is the same constant-product idea applied to pipes filling a tank.
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