1 foot= 12 inches
STEP 1 
Multiply 3 feet by 12 inches per foot
= 3 feet * 12 inches per foot
= 36 inches
STEP 2
Add inches in 3 feet with the additional 11 inches 
= 36 inches + 11 inches
= 47 inches 
ANSWER: Ian is 47 inches tall so he is not tall enough to ride. 
Hope this helps!  :)