Question 43

Select the option which is related to fifth number in the same way as second number is related to first number and fourth number is related to third number.
(Note: Numbers must be operated on whole numbers, without dividing them into their constituent digits. For example on 13 - 13 arithmetic operations like addition/subtraction/multiplication etc. can be done in 13. Dividing 13 into 1 and 3 and then doing arithmetic operations on 1 and 3 is not allowed.)

20 : 30 :: 42 : 56 :: 72 : ?

Solution

In each set, the consecutive multiples of the first numbers are added by 1 and again make the product to obtain the second number.
20 : 30

20 = $$5\times4$$ ==> $$(5+1)\times(4+1)$$ = $$6\times5$$ = 30

42 : 56

42 = $$7\times6$$ ==> $$(7+1)\times(6+1)$$ = $$8\times7$$ = 56

72 : ?

72 = $$9\times8$$ ==> $$(9+1)\times(8+1)$$ = $$10\times9$$ = 90

Hence '90' will come in place of the question mark.

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