Answer:
full page = 8
half page = 5
Step-by-step explanation:
two equations can be derived from the question
40x + 75y = 800 equation 1
x + y = 13 equation 2
x = number of half page advertisement
y = number of full page advertisement
multiply equation 2 by 40
40x + 40y = 520 equation 3
subtract equation 3 from 2
280 = 35y
y = 8
substitute for y in equation 2
x + 8 = 13
x = 13 - 8
x = 5