500
On Monday, you bike eight miles, take a break and then bike 10 more miles. On Tuesday, you repeat this. On Wednesday, you bike six miles less than on Tuesday. But, on Thursday, you manage to go 11 miles further than Monday. How far do you need to ride on Friday if you are going to make the 100 mile trip?
Monday = 8 + 10 = 18
Tuesday= 8+10= 18
Wed= 18-6 = 12
Thursday = 18+11= 29
So far: 18+18+12+29= 77 miles
To get to 100, you need 100-69 or 23 miles.