use mult and add to figure out fractions

2/10  of 20=10   2 x 10 =20

5\1 of 25=5        5 x 1=5

5\6 of 30=5       5 x 6=30

2\4 of 40=20    2x20=40

1\3 of 21=7       7x3=21

2\3 0f 15=10    2x5=10