If you want to easily find the solution you can use your calculator. Just divide 77 by 110 and see the result. 77/110 = 0.7 = 7/10 = 70/100 = 70% 100% - 70% = 30%
Explanation 110*x=77 x=77/110, but the answer is not a percent So 77/110 = ?/100 => ?=77*100/110=70 => 70% But since 70% does not represent a percentage decreasement, we need to subtract it from the 100% 100% - 70% = 30%