Answer:
40% of espresso was single-shot.
Explanation:
To make a percentage, you need to think of a fraction first. The denominator is always 100 in a percent. If my answer is 40%, the fraction would be 40/100. If there was 6 double shots and 4 single shots, those two together would make 10. 10 x 10 is 100. So really, you just have to multiply 4 by 10 to get the percentage.
Hope this helped, if your confused, just ask me :]
-Nific 0.9