Answer: 2 whole times (with remainder 10)
List out the multiples of 16 and we see that we have this list: 16, 32, 48, ...
We stop the moment we have a number larger than 42. So we have 16*2 = 32 showing how 16 goes into 42 exactly 2 whole times. If you had 42 cookies and 16 friends, then each friend gets 2 cookies. You'll have 10 left over since 42 - 32 = 10
Shortcut: If you want, you can use your calculator to say 42/16 = 2.625 where the "2" before the decimal point is the whole part and the "625" after the decimal is the fractional part. Multiply 0.625 with 16 and we end up with 0.625*16 = 10 which is the proper remainder.
Below is an attached image of what it would look like if you used long division.