This is a School Mathematics Study Group (SMSG) probability text for primary school students. The main goal of the text is to introduce some basic ideas of probability. The text is written at the level of a child's intuitive knowledge of what constitutes "an equal chance" or "a better chance." The authors suggest that there are certain fringe benefits to be derived from the unit: (1) it shows children that mathematics can be fun; (2) it promotes systematic thinking; (3) arithmetical skills may be practiced and reinforced; and (4) it provides opportunities for independent investigations. Chapter topics include: (1) certainty and uncertainty; (2) comparing the likelihood of various events; (3) combining events - this one and that one; (4) combining events - this one or that one; (5) number of possibilities; (6) combinations of two things; (7) combinations of three things or four things; (8) ordered arrangements; (9) arrangements and probability; and (10) repeated trials. (MP)