Suppose the old price per mango was p and you could buy x mangoes for
$75 at the old price.  Then

xp = 75

The price per mango was reduced by 20%, therefore the new price 
per mango is only 80% of the old price per mango, or 0.8p
and now x+5 mangos can be bought for $75, so 0.8p(x+5) = 75

So the system of equations is



Solve that and get the old price p = $3.75,   and the number
of mangoes x = 20 that you formerly could buy for $75.

The new price per mango is now .80($3.75) = $3

So a dozen mangoes now costs 12($3) or $36.

Edwin