Determine P(at least 2 heads).

The image shows a probability tree for 4 coin flips. There are 2^4 = 16 possible outcomes. We need to count the outcomes with at least 2 heads. These are: HHHT, HHTH, HHTT, HTHH, HTHT, HTTH, THHH, THHT, THTH, TTHH. There are 11 such outcomes. The probability is the number of favorable outcomes divided by the total number of outcomes: 11/16 = 0.6875.