25000 blood donors were registered with Red Cross in a city. The number of donors increased at the rate of 10% every 6 months. Find the total time (in years) at the end of which the total number of blood donors becomes 33275.

Initial number of blood donors = 25,000

Final number of blood donors = 33,275

Rate of increase = 10% every 6 months

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Since the number increases by 10% every 6 months, this follows the compound growth pattern.

Using the formula: Final Amount = Initial Amount × $(1 + \frac{\text{Rate}}{100})^n$

where $n$ is the number of 6-month periods.

Substituting the values:

$33,275 = 25,000 × (1.1)^n$

$\frac{33,275}{25,000} = (1.1)^n$

$1.331 = (1.1)^n$

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To find $n$, check which power of 1.1 equals 1.331:

Power	Calculation	Result
$n = 1$	$(1.1)^1$	$1.1$
$n = 2$	$(1.1)^2$	$1.21$
$n = 3$	$(1.1)^3$	$1.331$ ✓

So $n = 3$ (meaning 3 periods of 6 months each)

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Converting to years:

Total time = $3 × 6$ months = $18$ months = $1.5$ years

Correct Option: 4