Class $7A$ has 50 students and Class $7B$ has 40 students. On a field trip 44% of $7A$ and 55% of $7B$ took part. How many students took part altogether?

Work out each class separately, because the two percentages are taken of different totals. 44% of $50=\frac{44}{100}\times 50=22$; 55% of $40=\frac{55}{100}\times 40=22$. Altogether 22 + 22 = 44 students. The trap in 99 is adding the percentages (44 + 55) — percentages of different quantities can never simply be added. 49.5 and 39.6 apply just one of the rates to the combined 90 students.