Let
x-----------> mass a grain of salt
y-----------> mass a grain of sand
r----------> ratio mass a grain of salt than mass a grain of sand
we know that
x=6.5 × 10–2 gram
y=2.6 × 10–3 gram
r=x/y----------> r=[6.5 × 10–2]/[2.6 × 10–3]----------> r=25
the answer is
the mass of the grain of salt is 25 times greater than the mass of the grain of sand