King David (Dawid Hammelekh 'a"h), as we know, instituted that we have to recite 100 Berakhoth (blessings) every day. During the week that's not a problem. With all the Tefilloth (prayers) and food that we eat, we soon have a hundred blessings.
On Shabbath, however, it is a little more difficult because the 'Amidah has far less blessings than it does during the week. We make it up with Besameem (good smelling items), that we recite blessings on and extra fruits that we eat.
The real problem is on a Ta'anith (fast day), not only the ones that are both night and day, but even the ones like 'Asarah BeTebeth (the tenth of Tebeth) and Shib'ah 'Asar BeTammuz (the seventeenth day of Tammuz), on which we fast just during the day itself. This is because there are no foods eaten, and a lot less blessings are recited. So what is the solution?
In Ben Ish hai, Parashath Balaq it says the following: When you hear people reciting the blessings on the Torah, you should listen to them carefully and answer Amen. Since we have no other option, this can count towards the blessings that we are lacking. However, in order for that to be so, we must be particular to listen to the blessings from the beginning till the end.
(See Ben Ish Hai, Shana Aleph, Parashath Balaq, Oth Aleph)
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