CPU Memory Hierarchy: Calculating Average Memory Access Time
Your formulas look fine to me, but there's an error in the numbers:
$$
T_{DRAM} = (0.8)(60 ns) + (0.2)(7 ms) \\
T_{DRAM} = 1.448 \mu s
$$
This is incorrect, with \$0.2 * 7\$ ms in the sum, the answer should be at least 1.4 ms, not µs.

Note that 1 ms = 1000 µs = 1 000 000 ns.

Compare:

$$
T_{DRAM} = (0.8)(60 ns) + (0.2)(7000 ns) \\
T_{SRAM} = (0.8)(5 ns) + (0.2)(T_{DRAM}) \\
T_{SRAM} = 293.6 ns
$$

with:

$$
T_{DRAM} = (0.8)(60 ns) + (0.2)(7000000 ns) \\
T_{SRAM} = (0.8)(5 ns) + (0.2)(T_{DRAM}) \\
T_{SRAM} = 280013.6 ns
$$

If you substitute \$T_{DRAM}\$ in the \$T_{SRAM}\$ expression, you get:

$$
T_{SRAM} = (0.8)(5 ns) + (0.2)(0.8)(60 ns) + (0.2)(0.2)(7000000 ns)
$$

The expression you quoted from the book is slightly different (only one \$(0.2)\$ in the last term of the sum). That looks like either an error in the book, or in your quote.