Solve this:

Dear Student,

Let the number of candidates that appeared from school 3 be x

Average score of candidates of all schools = 
$\frac{\mathrm{score} \mathrm{of} \mathrm{candiates} \mathrm{in} \mathrm{school} 1+\mathrm{score} \mathrm{of} \mathrm{candiates} \mathrm{in} \mathrm{school} 2+\mathrm{score} \mathrm{of} \mathrm{candiates} \mathrm{in} \mathrm{school} 3+\mathrm{score} \mathrm{of} \mathrm{candiates} \mathrm{in} \mathrm{school} 4}{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{students} \mathrm{in} \mathrm{all} \mathrm{schools} }$
Now, 
Score of candidates in school 1 = Average Score * No. candidates
                                                  = 75 * 60
                                                 = 4500
Score of candidates in school 2 = 80*48
                                                  = 3840
Score of candidates in school 3 = 55 *x
                                                  = 55x
Score of candidates in school 4 = 50*40
                                                  = 2000
Therefore,

66 = $\frac{4500+3840+55\mathrm{x}+2000}{60+48+\mathrm{x}+40}$
66 * (148+x) = 10340 + 55x
66*148 + 66x = 10340 + 55x
11x = 10340 - 66*148
x = 940 - 6*148
x = 940 - 888
x = 52

Regards