Question 835139
6m+20c<=240,
m<=4c
6*4c+20c<=240

44c<=240
11c<=60
c<=60/11 <=5.45 
5 chapters for 100 minutes
140 minutes for math problems 
23 math problems

check
5*20=100 minutes
6*23=138 minutes
100+138 =238 minutes which is 2 minutes less than 4 hours.
5*4=20 
23 >20 so there are more than 4 math problems for every chapter read.