How many positive integers less than 40 are relatively prime to both 7 and 10?

Two integers are relatively prime if they share no common positive factors (divisors) except 1

 

for 7    1 2 3 4 5 6 8 9 10 11 12 13     15  16 17 18 19 20   22 23 24 25 26 27     29  30 31 32 33 34    36 37 38 39

USing the above list 

for 10: 1    3             9      11      13                17      19             23                27      29      31      33              37       39     count them up !