Hey Mousa,
heres the answer to ur question........
Limiting friction= mew * 2N(2times normal contact foce as she is using 2 fingers)
20 = 0.3*2N
20/0.3=2N
2N=66.67
N=33.33N
Hence the horizontal force she needs to apply to prevent the rice fromslipping is 33.33N
Hope this helped.........