The average of 5, 25, and 45 is:
(5 + 25 + 45) / 3 = 75/3 = 25
25 is 10 more than
15, which gives us the average involving our unknown amount:
(10 + 30 + x) / 3 = (40+x)/3 = 15
==> 40+x = 45
==> x = 5
Answer A.
_________________
GRE / GMAT Tutor London and Online since 2005. Writer, editor, contributor for test-prep materials.
Struggling with Permutations and Combinations (Combinatorics?). I've put loads of free resources here:
https://privategmattutor.london/gmat-combinatorics-ultimate-guide-to-gmat-permutations-and-combinations/As ever, feel free to get in touch for impartial GRE / GMAT advice or to enquire about lessons at:
https://privategmattutor.london