Multiply 70 by something like 12 first since we know it will still be a 3 digit number:
70 * 12 = 840
Now, we can slowly increase it until it gets as high as it can without becoming 4 digits:
70 * 13 = 910
70 * 14 = 980
Because multiplying any higher will yield a number with over 3 digits, and 70 is not a 3 digit number (subtract 1 from the 14), there are 13 3 digit numbers divisible by 7.
Hope this helps! :)
