Hello Walker.
Lets say x represents amount of 60% and
1000 - x represents 40% of mixture. (total is 1000ml)
the equation should be
x*60% + (1000- x)*40% = 1000*48% (you can cancel all of the percents by multiplying each by 100)
60x +(1000-x)*40 = 1000*48
60x +40000 - 40x = 48000 (combine x)
20x +40000 = 48 000. (cancel 40000)
20x = 8000. (cancel 20)
x = 400
400ml of 60%; 1000-400 = 600ml of 40%
hope that helps