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In a survey, it was found that 10% of the students who are susceptibl [#permalink]
2
Let percent of students who are susceptible to TB be x

percent not susceptible is y


x + y = 100..............1


Less than 20 and susceptible = 0.1x


Greater than or equal to 20 and susceptible = 9%


Greater than or equal to 20 and not susceptible = 0.4y


Therefore

0.1x + 9 = x

0.1x - x = -9

-0.9x = -9

x = 10%

So

10 + y = 100
y = 90%

Since greater than or equal to 20 and not susceptible = 0.4y

That gives us 0.4 x 90 = 36%

Percent of students greater than or equal to 20 = 9 + 36 = 45%

Required percent = 36/45 x 100 = 80%

Answer E
Adewale Fasipe, quant instructor from Lagos Nigeria.
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