# Permutations

### Do you have questions about Permutations?

How many two-digit numbers can be formed from the numbers 1,2,3,4 is repetirion is allowed

Krishna Laxman
Mathematics Trainer and Educational consultant

Two digit even number :  A numbers between 10 and 99 (including both) and one's digit should be even.

In our case ,the ten's digits can be filled in 5 ways and the one's digit can be filled in 2 ways

so totally $5\cdot2$ = 10 .

Question

### How do you know when to use $^nP_r$ and when to use $^nC_r$ in Permutations and Combinations problems? Please show some examples to make this point clear.

Each time a coin is tossed the two possible outcomes are heads (H) or tails (T). So with three tosses the possible outcomes are $2 \times 2 \times 2 = 8$ outcomes. They are: HHH, HHT, HTH, HTT, THH, THT, TTH, TTT.