The height need to be contributed to the 12 of 141.5cm children to reach 143cm in average are:

143-141.5 = 1.5 cm

1.5cm x 12 = 18cm

The 18 cm is from the rest of children who are 145cm in average.

They are contributing 145 cm – 143 cm = 2cm in average towards the overall group’s height.

Therefore, there are 18 ÷ 2 = 9 children remain.

At first, there were 9 + 12 = 21 children in total.