9 - (10d) = 11
Add 9 to both sides.
10d = 20
Divide both sides by 10.
d = 2
The answer is d = 2.
The probability that the first random choice will be a matho-file and not a -phobe is 18/23. The probability of the second one is 17/22, and for the third one it's 16/21. The probability of all 3 is 4,896/10,626 = (18x17x16)/(23x22x21). That's 46.1% (rounded).