The formula to use is the nCr combination formula
n C r = (n!)/(r!*(n-r)!)
where the exclamation marks indicate factorial notation
Plug n = 10 and r = 2 into the formula
n C r = (n!)/(r!*(n-r)!)
10 C 2 = (10!)/(2!*(10-2)!)
10 C 2 = (10!)/(2!*8!)
10 C 2 = (10*9*8!)/(2!*8!)
10 C 2 = (10*9)/(2!)
10 C 2 = (10*9)/(2*1)
10 C 2 = 90/2
10 C 2 = 45
Answer: 45
