SPF10 only blocks off 93% of UVB (which is responsible for sun burn) If it normally takes 10mins for your husband to get burn under the sun, applying a SPF10 sunscreen will give you approximately 100mins of sun protection. (10mins x SPF10) however, the sunscreen you used (SPF50) blocks off ~98% of UVB so you can say that it blocks off more UVB and lasts 5 times longer than the SPF10 sunscreen your husband was using.
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