The result of multiplying (or adding, subtracting, or dividing) a float with a double is a double, and Java won't store a double into a float without a "cast". A cast is just a hint to the compiler that it's OK to convert the type of a value.
aFloat = aFloat * aDouble;
you have to write
aFloat = (float) (aFloat * aDouble);
Now, I should mention that in casting, there are potential problems; in particular, doubles can hold values that floats can't hold, and the cast can give you a dramatically different value (the too-large part will be simply truncated, giving you a different number all together.) Be careful, and consider simply using the same type everywhere.