nCr = n ! r ! ( n r ) ! calculates the number of ways you can choose exactly r successes in n trials. Use a calculator or calculate by hand the number of ways to choose exactly 3 success in 5 trials. On the TI 83/84: Type in the number of trials. Select MATH, scroll right to highlight PRB, select nCr (or aCb) Type in the number of successes. Enter