Hah! So, my educated guess was pretty much spot on. It's not that hard to figure out - most of Africa is illiterate (regardless of religion, so the muslim numbers there will stack up) and then you have a significant number of muslims in Pakistan, India and Bangladesh - a region that is still struggling to get out of abject poverty!
42% of Muslims in India are illiterate - 82mil out of 195mil
36.4% of Hindus in India are illiterate - so, around 360mil out of a billion.
You can see how the numbers add up.