#### Answer

Mary needs to give $0.3$ grains.

#### Work Step by Step

RECALL:
$P=BR$ where $B$=base, $P$=part, and $R$= rate or percent
The given problem has:
$B=0.75$ grains
$R=40\%$
Solve for $P$ using the formula above to obtain:
$P=BR
\\P=40\% \times 0.75
\\P=0.40 \times 0.75
\\P=0.3 \text{ grains}$
Thus, Mary needs to give $0.3$ grains.