Bunuel wrote:

GMAT CLUB'S CHALLENGE QUESTIONS:

How many positive four-digit integers have their digits in ascending order from thousands to units?

A. 6

B. 7

C. 84

D. 126

E. 210

First Digit - A

Second Digit - B

Third Digit - C

Fourth Digit - D

A can be 1-6

B can be 2-7

C can be 3-8

D can be 4-9

A=1; B = 2; C = 3 ; D = 6 options

A=1; B = 2; C = 4; D = 5 options

A = 1; B=2; C = 5; D = 4 options

A = 1; B=2; C = 6; D = 3 options

A = 1; B=2; C = 7; D = 2 options

A = 1; B=2; C = 8; D = 1 options

So When A =1 and B=2 then we have 21 options

WHen A = 1 and B=3 then we have 15 options

When A =1 and B=4 then we have 10 options

When A = 1 and B=5 then we have 6 options

When A = 1 and B=6 then we have 3 options

When A = 1 and B=7 then we have 1 option

So When A =1 we have 56 options

WHen A = 2 we have 35 options

When A = 3 we have 20 options

When A = 4 we have 10 options

When A = 5 we have 4 options

When A = 6 we have 1 option

Alltogether we have 56+35+20+10+4+1 = 126 options

D

Please correct me if i am wrong

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