If Fred weighs 100kg,and his total blood vol.is 7%, his heart pumps
his total blood vol. within 90secs. and renal blood flow is 20% of
cardiac output what is the volume of blood flowing thru his kidneys
each minute?

You've provided all the info right you need in front of you; just do the math.

100 kilos times 7% (that's 0.07) gives you blood volume in litres.

Multiply that by 20% (0.20) to get renal flowthrough.

Then multiply that by 60/90 (or 2/3) to get rate per minute.

Now get to work!