Hello!
I'll start w/ 18.74 to round. To round 18.74, the 4 wouldn't have any effect raising the 7's value, since anything 4 and below won't increase the number's value. Now you just have 18.7. The 7 will increase the 8's value to a 9, since anything 5 and up will increase the rounded number's value. So 18.74 is now rounded up to 19.
For 41.49, you'll round the 4 to a 5, since 9 will increase the value of the 4. Now you have 41.5. The newly rounded 5 will then increase the 1's value, making it a 2. So now 41.49 is now rounded up to 42.
You can now estimate the sum by adding 19 w/ 42 to get an estimate of 61!
I hope this helps!!!