To find the whole when a fraction of it is known: divide by the fraction (multiply by its reciprocal). Then find the required fraction of the whole.

Step 1: Find the original number. If 2/3 of it is 64, then the number = ?
Whole number	=	\(64 \div \dfrac{2}{3} = 64 \times \dfrac{3}{2} = 64 \div 2 \times 3 = 32 \times 3 = 96\)
Step 2: Find 5/8 of this number.
\(\dfrac{5}{8}\) of 96	=	\(96 \div 8 \times 5 = 12 \times 5 = 60\)