When you have a break in your presentation, for example a 15 minutes coffee break or one hour lunch break you should be very clear towards your audience about the length of the break.
If you just say the length of the break once, you are almost guaranteed that not everyone will hear you and therefore this could lead to confusion, audience members not being back on time after the break.
To avoid such situation you should indicate (A) the length of the break and (B) the time when you start again.
For example “We will now have a 20 minutes break which means that we will start again at four o’clock. So, 20 minutes break and back at four”.
By means of this you have said it actually 4 times, which should be enough for all members of your audience to understand. You may also include a slide or note on a large flip-over chart on which you indicate the time until there is a break.