There are 18 very tall boys, which constitute 3/4 of the total number of boys. The total number of boys is 2/3 of the total number of students in the class. Let the total number of students in the class be denoted by $S$.

Step 1: Find the total number of boys. We know that 3/4 of the total number of boys is 18, so: $(\frac{3}{4}) \times \text{total boys} = 18$ To find the total number of boys: total boys = $(18 \times 4) / 3 = 24$

Step 2: Find the total number of students. The total number of boys is 2/3 of the total number of students. So: $(\frac{2}{3}) \times \text{total students} = \text{total boys}$ $(\frac{2}{3}) \times S = 24$ Now, solve for $S$: $S= (24 \times 3) / 2 = 36$

Step 3: Find the number of girls in the class. The total number of students is 36, and the number of boys is 24. So, the number of girls is: girls = total students - total boys girls = $36 - 24 = 12$