log

2*10^(2-x) =5      divide both sides by 2

10^(2 - x)  =  5/2      take the base 10 log of eac side

log 10^(2 - x)  = log (5/2)     and we can write

(2 - x)  log 10   = log (5/2)      since log 10 = 1, we can ignore this, so

2 - x  = log (5/2)      add x to both sides and subtract log(5/2) from both sides

2 - log (5/2)   = x   ≈ 1.602