Plug n = 18 and r = 6 into the nCr combination formula
n C r = (n!)/(r!*(n-r)!) 18 C 6 = (18!)/(6!*(18-6)!) 18 C 6 = (18!)/(6!*12!) 18 C 6 = (18*17*16*15*14*13*12!)/(6!*12!) 18 C 6 = (18*17*16*15*14*13)/(6!) 18 C 6 = (18*17*16*15*14*13)/(6*5*4*3*2*1) 18 C 6 = 13366080/720 18 C 6 = 18564
90 words in 4 minutes is the same as 45 words in 2 minutes because you divide both sides by 2. 2 goes into 10, 5 times, so 45x5=225. You can expect her to type 225 words in 10 minutes.
