Question : Directions: Select the option that is related to the fourth number in the same way as the first number is related to the second number and the fifth number is related to the sixth number.
15 : 210 :: ? : 90 :: 20 : 380
Option 1: 10
Option 2: 8
Option 3: 18
Option 4: 12
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Correct Answer: 10
Solution :
Given:
15 : 210 :: ? : 90 :: 20 : 380
Like, 15 : 210→15 × (15 – 1) = 15 × 14 = 210
And, 20 : 380→20 × (20 – 1) = 20 × 19 = 380
Similarly, follow the same pattern for 90 : ?→Let the missing number be a.
⇒ a × (a – 1) = 90
⇒ a
2
– a – 90 = 0
⇒ a
2
– 10a + 9a – 90 = 0
⇒ a(a – 10) + 9(a – 10) = 0
⇒ (a – 10)(a + 9) = 0
⇒ a = 10 or –9
So, 10 is related to 90. Hence, the first option is correct.
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