Question 730607
Letting x = pounds of sand to add to reach 35% sand,


{{{highlight((642*0.29+x)/(642+x)=0.35)}}}, the fundamental ratio for the adjustment, and as you wanted, using decimal fractions instead of "percents" units.
{{{642*0.29+x=0.35(642+x)}}}
{{{642*0.29+x=0.35*642+0.35x}}}
.
{{{642(0.29-0.35)=(0.35-1)x}}}
{{{642(0.35-0.29)=(1-0.35)x}}}
.
{{{x=(0.06/0.65)642}}}
{{{highlight(x=59)}}}



SOME COMMENTS:

The numerator in the ratio equation is the total sand present in the adjusted mixture.  The denominator in the ratio equation is the total amount of adjusted mixture.  The entire ratio, the left-hand member of the equation, must be equal to the specified 0.35, or 35%.

Since the percents given in the description only go to two significant figures, I kept my result answer to two significant figures also.  If we could allow for THREE significant figures, our answer should probably be more like 59.3 pounds of sand.