Alright. So this example asks us to imagine 15 neurons synapse onto 1 postsynaptic neuron. At the trigger zone, 13 of the neurons produce EPSPs of 2 millivolts each, and the other 2 produce IPSPs of 3 millivolts each. The threshold for the postsynaptic cell is negative 55. We know the threshold, right? And in this scenario, we are asked whether an action potential would be produced. And then they just kind of remind us that we are currently at the resting potential at negative 70. So, as we've been discussing summation, let's do some adding:

• We've got 13 neurons that are sending us EPSPs of 2 millivolts each. Remember, EPSPs are positive. They're going to make our membrane more positive, and IPSPs are going to make our membrane more negative.
• These 13 EPSPs of 2 millivolts each are going to get us a total change of 26 millivolts, positive 26 millivolts.
• These 2 IPSPs that are 3 millivolts each are going to get us a total of negative 6 millivolts.

So we just add those together: 26 minus 6, which gives us a net change from all 15 of these of 20 millivolts, and that is positive. Now, we're starting at rest, so we're at negative 70. If we add 20 millivolts to negative 70, we are going to end up with negative 50. Now the threshold is negative 55, and we have gotten even more positive than that; we have surpassed the threshold. In this case, an action potential would indeed happen. We have hit and even surpassed the threshold. So there you go.