use benchmarks to estimate 12.54 +7.98
since 12.54 rounds to 12.5, and 7.98 rounds to 8, 12.5 plus 8 would equal 20.5. the estimated answer would be around 20, 20.5 or 21
12.54 ≈ 12.57.98 ≈ 812.5 + 8 = 20.520.5, hope i helped