CONCEPT: The question tests on the basics of equation solving.
Let their be A people in group A and B be the number of people in group B.
Thus A+B=50 .....(1)
Also 80A + 90 B =4270 ....(2) (The cost/costume for group A is $80 and for B its $90)
Solving both (1) and (2), we have A=23 and B=27
Thus, cost for group B = 90x27 = $2630 (OPTION E)