Solution: It is known that each class plans to plant 39 trees on average, and there are 6 classes in Grade Three.

The number of trees that can be planted = the average number of trees planted in each class × the number of classes.

=39×6

=234 trees

Extended data

Multiplication is a shortcut to add up the same numbers. In ancient China, multiplication was carried out by several chips. There are three layers in calculating multiplication: the upper layer is the multiplicand, the middle layer is the product and the lower layer is the multiplier. Multiply the multiplicand by the largest bit of the multiplier, remove the calculation of this bit after multiplication, multiply it by the second bit, and add the numbers on the corresponding bits of the two products until the multiplication is over.

1494, the Italian mathematician Bacceli (1445- 15 14) introduced eight kinds of multiplication. The first multiplication is exactly the same as the commonly used pen multiplication, and the sixth multiplication is grid multiplication. This method was introduced to China in the15th century, and it is also called tapestry because of its graphic resemblance to tapestry (see the figure below).

Multiplication principle: If the dependent variable f is in direct proportion to the independent variable X 1, X2, X3...Xn, and each independent variable is qualitatively different, if any independent variable f is missing, it will lose its meaning, which is called multiplication.