Hey there! I'm happy to help!
First, we need to find 60% of 140. This is how many boxes she sold to the people her dad works with. 60% as a decimal is 0.6 (0.6 is 60% of 1, one represents the whole, or total).
0.6×140=84
We know that 1/4 of these had mint cookies. 1/4 is 25%, so let's find 25% of 84!
0.25×84=21
Therefore, Lauren sold 21 boxes of mint cookies to people her dad works with.
Have a wonderful day! :D