Write $5\times 10^{-4}$ as an ordinary decimal number.

$5\times 10$⁻⁴ means $5÷10^4=5/10000=0.0005$ — the decimal point moves 4 places to the LEFT. Moving it only 3 places gives the trap 0.005; moving it 5 places gives 0.00005; and treating the exponent as +4 (multiplying instead of dividing) gives 50000.