He can buy 7 plates leaving $20 for his savings
see the attached figure to better understand the problem
we know that
The equilateral triangle has three equal sides
so
in the equilateral triangle ABC

the height of the triangle is the segment BD
in the right triangle BCD
Applying the Pythagorean Theorem

solve for BD

substitute the values



therefore
<u>the answer is</u>
the height of the triangle is 
$0.70 per candy.
$78 divided by 112 gives $0.696, so by rounding off, you get 0.70