2/5 of 20 = 8.
3/10 of 20= 6.
8+6=14. So 20-14= 6
She needs 14 pints, so you subtract 6 from that. Your final answer should be 8 pints.
Dividing is changed to multiplication by inverting the second fraction. So:
1/3=2/6 so
6=36/6 and you add 2/6 and 36/6 to get 38/6 or 19/3
Or 38 sixths
