Simplification: $25^{2.7} \times 5^{4.2} \div 5^{5.4} = ?$

We can rewrite the equation as:

$(5^2)^{2.7}\times5^{4.2}\div5^{5.4}$

$=5^{5.4}\times5^{4.2}\div5^{5.4}$

We know that powers add when multiply and subtract when divided.

$=5^{9.6}\div5^{5.4}=5^{4.2}$